Bump coverage from 5.5 to 6.0.1

Bumps [coverage](https://github.com/nedbat/coveragepy) from 5.5 to 6.0.1.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@coverage-5.5...6.0.1)

---
updated-dependencies:
- dependency-name: coverage
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements-dev.txt

@@ -5,7 +5,7 @@ isort==5.9.3
 flake8==3.9.2
 pydocstyle==6.1.1
 docformatter==1.4
-coverage==5.5
+coverage==6.0.1
 autoflake==1.4
 tox==3.24.4
 pur==5.4.2