Bump flake8-docstrings from 1.5.0 to 1.6.0 (#150)

Bumps [flake8-docstrings](https://gitlab.com/pycqa/flake8-docstrings) from 1.5.0 to 1.6.0.
- [Release notes](https://gitlab.com/pycqa/flake8-docstrings/tags)
- [Changelog](https://gitlab.com/pycqa/flake8-docstrings/blob/master/HISTORY.rst)
- [Commits](https://gitlab.com/pycqa/flake8-docstrings/compare/1.5.0...1.6.0)

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

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

requirements_test.txt

@@ -1,7 +1,7 @@
 aresponses==2.1.4
 coverage==5.5
 flake8==3.8.4
-flake8-docstrings==1.5.0
+flake8-docstrings==1.6.0
 isort==5.8.0
 mypy==0.812
 pylint==2.7.2