Bumps pytest-cov from 2.11.1 to 2.12.1.

Sourced from pytest-cov's changelog.

2.12.1 (2021-06-01)

• Changed the toml requirement to be always be directly required (instead of being required through a coverage extra). This fixes issues with pip-compile (pip-tools#1300 <https://github.com/jazzband/pip-tools/issues/1300>). Contributed by Sorin Sbarnea in [#472](https://github.com/pytest-dev/pytest-cov/issues/472) <https://github.com/pytest-dev/pytest-cov/pull/472>.
• Documented show_contexts. Contributed by Brian Rutledge in [#473](https://github.com/pytest-dev/pytest-cov/issues/473) <https://github.com/pytest-dev/pytest-cov/pull/473>_.

2.12.0 (2021-05-14)

• Added coverage's toml extra to install requirements in setup.py. Contributed by Christian Riedel in [#410](https://github.com/pytest-dev/pytest-cov/issues/410) <https://github.com/pytest-dev/pytest-cov/pull/410>_.
• Fixed pytest_cov.__version__ to have the right value (string with version instead of a string including __version__ =).
• Fixed license classifier in setup.py. Contributed by Chris Sreesangkom in [#467](https://github.com/pytest-dev/pytest-cov/issues/467) <https://github.com/pytest-dev/pytest-cov/pull/467>_.
• Fixed commits since badge. Contributed by Terence Honles in [#470](https://github.com/pytest-dev/pytest-cov/issues/470) <https://github.com/pytest-dev/pytest-cov/pull/470>_.

• 67d49b9 Bump version: 2.12.0 → 2.12.1
• 3685acb Use 2019 image for pypy.
• df20c97 Looks like I got the images wrong.
• 17252e0 Remove unnecessary pin.
• ac7cc91 Update changelog.
• 34edfa1 Avoid using toml as extra (#472)
• edc60bb Document show_contexts (#473)
• 297965e Update appveyor envs. Workaround pypy install problem.
• 9692bad Bump version: 2.11.1 → 2.12.0
• f533121 Update changelog.
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