Bump pydocstyle from 5.1.1 to 6.0.0 (#406)

Bumps [pydocstyle](https://github.com/PyCQA/pydocstyle) from 5.1.1 to 6.0.0.
- [Release notes](https://github.com/PyCQA/pydocstyle/releases)
- [Changelog](https://github.com/PyCQA/pydocstyle/blob/master/docs/release_notes.rst)
- [Commits](https://github.com/PyCQA/pydocstyle/compare/5.1.1...6.0.0)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>
Co-authored-by: Pierre Ståhl <pierre.staahl@gmail.com>

requirements_test.txt

@@ -2,7 +2,7 @@ black==20.8b1
 codespell==2.0.0
 flake8==3.8.4
 mypy==0.812
-pydocstyle==5.1.1
+pydocstyle==6.0.0
 cryptography==3.2
 pylint==2.7.2
 pylint-strict-informational==0.1