Upgrading to oauth2client 2.0.

…ion (#1479)

* chore: fix snippet metadata filename and add placeholder library version

* fix: Typo when setting the state for the pickle deserializer.

Previously googleapis/python-spanner#1452
introduced changes to support python spanner client against spanner
experimental host endpoints over insecure communication

This PR extends those changes to support python spanner client
connections to experimental host endpoints over TLS / mTLS connections
as well. It also includes changes to run Integration Tests against
experimental hosts across all 3 modes of network communication
(plain-text, TLS, mTLS)

To run IT tests against experimental host set below variables
```
export SPANNER_EXPERIMENTAL_HOST=localhost:15000
```
For tls/mTLS set below additonal variables:
- (mTLS/TLS)
```
export CA_CERTIFICATE=/tmp/experimental_host/ca-certificates/ca.crt
```
- (mTLS)
```
export CLIENT_CERTIFICATE=/tmp/experimental_host/certs/client.crt
export CLIENT_KEY=/tmp/experimental_host/certs/client.key
```

Then we can run below command to tigger the tests:
```
python -m pytest -v -s --disable-warnings  tests/system/
```

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: gcf-merge-on-green[bot] <60162190+gcf-merge-on-green[bot]@users.noreply.github.com>

PR created by the Librarian CLI to initialize a release. Merging this PR
will auto trigger a release.

Librarian Version: v0.0.0-20260331223231-14b1b9186e21
Language Image:
us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator@sha256:234b9d1f2ddb057ed7ac6a38db0bf8163d839c65c6cf88ade52530cddebce59e
<details><summary>google-cloud-spanner: v3.64.0</summary>

##
[v3.64.0](google-cloud-spanner-v3.63.0...google-cloud-spanner-v3.64.0)
(2026-04-01)

### Features

* use inline begin to eliminate BeginTransaction RPC (#1502)
([00d5f8e6](00d5f8e6))

* add TLS/mTLS support for experimental host (#1479)
([0c4c5da](0c4c5da3))

* add Client Context support to options (#1499)
([0da5f78](0da5f785))

* Exposing total CPU related fields in AutoscalingConfig
(PiperOrigin-RevId: 845819318)
([9d2338a](9d2338a6))

* include cache updates into the ResultSet response (PiperOrigin-RevId:
865546011)
([9d2338a](9d2338a6))

* add a ClientContext field to Spanner requests (PiperOrigin-RevId:
853323071)
([9d2338a](9d2338a6))

* include cache updates and routing hint into BeginTransaction and
Commit request/response respectively (PiperOrigin-RevId: 878019893)
([9d2338a](9d2338a6))

* add Secure Parameters to the ClientContext (PiperOrigin-RevId:
853323071)
([9d2338a](9d2338a6))

* implement native asyncio support via Cross-Sync (#1509)
([f822fd7](f822fd7c))

### Bug Fixes

* add threading import and _resource_info to snapshot.py
([659034a](659034aa))

* restore implicit database_dialect reload in sync client (#1537)
([67f5104](67f51043))

* resolve async deadlock in multiplexed session manager (#1520)
([c8f936b](c8f936b0))

### Documentation

* mark new AsyncIO APIs as experimental (#1533)
([3909c04](3909c045))

* A comment for field `transaction_tag` in message
`.google.spanner.v1.RequestOptions` is changed (PiperOrigin-RevId:
865546011)
([9d2338a](9d2338a6))

* A comment for field `routing_hint` in messages
`.google.spanner.v1.ResultSet` and `.google.spanner.v1.PartialResultSet`
are changed (PiperOrigin-RevId: 878019893)
([9d2338a](9d2338a6))

* Update high_priority_cpu_utilization_percent in AutoscalingConfig to
be Optional and clarify its behavior when not specified
(PiperOrigin-RevId: 845819318)
([9d2338a](9d2338a6))

* A comment for field `params` in message
`.google.spanner.v1.PartitionQueryRequest` is changed
(PiperOrigin-RevId: 865546011)
([9d2338a](9d2338a6))

* A comment in message
`.google.spanner.v1.TransactionOptions.ReadWrite.ReadLockMode` is
changed (PiperOrigin-RevId: 878019893)
([9d2338a](9d2338a6))

* A comment for field `commit_timestamp` in message
`.google.spanner.v1.BatchWriteResponse` is changed (PiperOrigin-RevId:
865546011)
([9d2338a](9d2338a6))

* minor update for Spanner Location API (PiperOrigin-RevId: 834841888)
([9d2338a](9d2338a6))

* A comment for field `param_types` in message
`.google.spanner.v1.PartitionQueryRequest` is changed
(PiperOrigin-RevId: 865546011)
([9d2338a](9d2338a6))

</details>