Updating Sphinx theme from default to classic.

* feat: OR Query implementation

Introduce new Filter classes:

FieldFilter
And
Or

Add "filter" keyword arg to "Query.where()"
The positional arguments in "Query.where()" are now optional.
UserWarning is now emitted when using "where()" without keyword args.

* Add test coverage

* Add system tests using transaction

* Add more complex system test

* Remove leftover comment

* Lint fix

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* Add system test demonstrating legacy where still works and returns value

* Remove debugging comment.

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Source-Link: googleapis/synthtool@7197a00
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:c43f1d918bcf817d337aa29ff833439494a158a0831508fda4ec75dc4c0d0320

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Closes #697.

Source-Link: googleapis/synthtool@7197a00
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:c43f1d918bcf817d337aa29ff833439494a158a0831508fda4ec75dc4c0d0320

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Using workload identity federation, applications can access Google Cloud resources from Amazon Web Services (AWS), Microsoft Azure or any identity provider that supports OpenID Connect (OIDC). Workload identity federation is recommended for non-Google Cloud environments as it avoids the need to download, manage and store service account private keys locally.

This includes a rollforward of the [previous reverted PR](googleapis/google-auth-library-python#686) and the [fix](googleapis/google-auth-library-python#686) to not pass scopes to user credentials from `google.auth.default()`.

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

* feat: add max_results to a few QueryJob methods

It is now possible to cap the number of result rows returned when
invoking `to_dataframe()` or `to_arrow()` method on a `QueryJob`
instance.

* Work around a pytype complaint

* Make _EmptyRowIterator a subclass of RowIterator

Co-authored-by: Dan Lee <71398022+dandhlee@users.noreply.github.com>

Supersedes #711.


## [2.21.0](https://www.github.com/googleapis/python-bigquery/compare/v2.20.0...v2.21.0) (2021-07-13)


### Features

* Add max_results parameter to some of the `QueryJob` methods. ([#698](https://www.github.com/googleapis/python-bigquery/issues/698)) ([2a9618f](https://www.github.com/googleapis/python-bigquery/commit/2a9618f4daaa4a014161e1a2f7376844eec9e8da))
* Add support for decimal target types. ([#735](https://www.github.com/googleapis/python-bigquery/issues/735)) ([7d2d3e9](https://www.github.com/googleapis/python-bigquery/commit/7d2d3e906a9eb161911a198fb925ad79de5df934))
* Add support for table snapshots. ([#740](https://www.github.com/googleapis/python-bigquery/issues/740)) ([ba86b2a](https://www.github.com/googleapis/python-bigquery/commit/ba86b2a6300ae5a9f3c803beeb42bda4c522e34c))
* Enable unsetting policy tags on schema fields. ([#703](https://www.github.com/googleapis/python-bigquery/issues/703)) ([18bb443](https://www.github.com/googleapis/python-bigquery/commit/18bb443c7acd0a75dcb57d9aebe38b2d734ff8c7))
* Make it easier to disable best-effort deduplication with streaming inserts. ([#734](https://www.github.com/googleapis/python-bigquery/issues/734)) ([1246da8](https://www.github.com/googleapis/python-bigquery/commit/1246da86b78b03ca1aa2c45ec71649e294cfb2f1))
* Support passing struct data to the DB API. ([#718](https://www.github.com/googleapis/python-bigquery/issues/718)) ([38b3ef9](https://www.github.com/googleapis/python-bigquery/commit/38b3ef96c3dedc139b84f0ff06885141ae7ce78c))


### Bug Fixes

* Inserting non-finite floats with `insert_rows()`. ([#728](https://www.github.com/googleapis/python-bigquery/issues/728)) ([d047419](https://www.github.com/googleapis/python-bigquery/commit/d047419879e807e123296da2eee89a5253050166))
* Use `pandas` function to check for `NaN`. ([#750](https://www.github.com/googleapis/python-bigquery/issues/750)) ([67bc5fb](https://www.github.com/googleapis/python-bigquery/commit/67bc5fbd306be7cdffd216f3791d4024acfa95b3))


### Documentation

* Add docs for all enums in module. ([#745](https://www.github.com/googleapis/python-bigquery/issues/745)) ([145944f](https://www.github.com/googleapis/python-bigquery/commit/145944f24fedc4d739687399a8309f9d51d43dfd))
* Omit mention of Python 2.7 in `CONTRIBUTING.rst`. ([#706](https://www.github.com/googleapis/python-bigquery/issues/706)) ([27d6839](https://www.github.com/googleapis/python-bigquery/commit/27d6839ee8a40909e4199cfa0da8b6b64705b2e9))

🤖 I have created a release \*beep\* \*boop\*
---
## [2.21.0](https://www.github.com/googleapis/python-bigquery/compare/v2.20.0...v2.21.0) (2021-07-14)


### Features

* add always_use_jwt_access ([#714](https://www.github.com/googleapis/python-bigquery/issues/714)) ([92fbd4a](https://www.github.com/googleapis/python-bigquery/commit/92fbd4ade37e0be49dc278080ef73c83eafeea18))
* add max_results parameter to some of the QueryJob methods ([#698](https://www.github.com/googleapis/python-bigquery/issues/698)) ([2a9618f](https://www.github.com/googleapis/python-bigquery/commit/2a9618f4daaa4a014161e1a2f7376844eec9e8da))
* add support for decimal target types ([#735](https://www.github.com/googleapis/python-bigquery/issues/735)) ([7d2d3e9](https://www.github.com/googleapis/python-bigquery/commit/7d2d3e906a9eb161911a198fb925ad79de5df934))
* add support for table snapshots ([#740](https://www.github.com/googleapis/python-bigquery/issues/740)) ([ba86b2a](https://www.github.com/googleapis/python-bigquery/commit/ba86b2a6300ae5a9f3c803beeb42bda4c522e34c))
* enable unsetting policy tags on schema fields ([#703](https://www.github.com/googleapis/python-bigquery/issues/703)) ([18bb443](https://www.github.com/googleapis/python-bigquery/commit/18bb443c7acd0a75dcb57d9aebe38b2d734ff8c7))
* make it easier to disable best-effort deduplication with streaming inserts ([#734](https://www.github.com/googleapis/python-bigquery/issues/734)) ([1246da8](https://www.github.com/googleapis/python-bigquery/commit/1246da86b78b03ca1aa2c45ec71649e294cfb2f1))
* Support passing struct data to the DB API ([#718](https://www.github.com/googleapis/python-bigquery/issues/718)) ([38b3ef9](https://www.github.com/googleapis/python-bigquery/commit/38b3ef96c3dedc139b84f0ff06885141ae7ce78c))


### Bug Fixes

* inserting non-finite floats with insert_rows() ([#728](https://www.github.com/googleapis/python-bigquery/issues/728)) ([d047419](https://www.github.com/googleapis/python-bigquery/commit/d047419879e807e123296da2eee89a5253050166))
* use pandas function to check for NaN ([#750](https://www.github.com/googleapis/python-bigquery/issues/750)) ([67bc5fb](https://www.github.com/googleapis/python-bigquery/commit/67bc5fbd306be7cdffd216f3791d4024acfa95b3))


### Documentation

* add docs for all enums in module ([#745](https://www.github.com/googleapis/python-bigquery/issues/745)) ([145944f](https://www.github.com/googleapis/python-bigquery/commit/145944f24fedc4d739687399a8309f9d51d43dfd))
* omit mention of Python 2.7 in `CONTRIBUTING.rst` ([#706](https://www.github.com/googleapis/python-bigquery/issues/706)) ([27d6839](https://www.github.com/googleapis/python-bigquery/commit/27d6839ee8a40909e4199cfa0da8b6b64705b2e9))
---


This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).

* ci(python): run lint / unit tests / docs as GH actions

Source-Link: googleapis/synthtool@57be0cd
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:ed1f9983d5a935a89fe8085e8bb97d94e41015252c5b6c9771257cf8624367e6

* exclude templated github actions

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>