Bump pytest-cov from 2.10.0 to 2.11.1

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.10.0 to 2.11.1.
- [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@v2.10.0...v2.11.1)

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