Bump pytest-cov from 2.12.1 to 3.0.0 (#6035)

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.12.1 to 3.0.0.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v2.12.1...v3.0.0)

---
updated-dependencies:
- dependency-name: pytest-cov
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/test.txt

@@ -7,7 +7,7 @@ mypy==0.910; implementation_name=="cpython"
 mypy-extensions==0.4.3; implementation_name=="cpython"
 proxy.py==2.3.1
 pytest==6.1.2
-pytest-cov==2.12.1
+pytest-cov==3.0.0
 pytest-mock==3.6.1
 re-assert==1.1.0
 setuptools-git==1.2