Bump pytest-cov from 2.7.1 to 2.12.0

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.7.1 to 2.12.0.
- [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@v2.7.1...v2.12.0)

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

Author: dependabot[bot]

requirements_dev_optional.txt

@@ -13,7 +13,7 @@ pymongo==3.11.4
 tox==3.14.0
 coverage
 flake8==3.9.2
-pytest-cov==2.7.1
+pytest-cov==2.12.0
 pytest-doctestplus==0.4.0
 h5py==2.10.0
 s3fs==0.5.1; python_version > '3.6'