Bump flake8 from 3.8.3 to 3.9.2 (#747)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.3 to 3.9.2.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.3...3.9.2)

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

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

Author: dependabot[bot]

requirements_dev_optional.txt

@@ -12,7 +12,7 @@ pymongo==3.11.4
 # optional test requirements
 tox==3.14.0
 coverage
-flake8==3.8.3
+flake8==3.9.2
 pytest-cov==2.7.1
 pytest-doctestplus==0.4.0
 h5py==2.10.0