Bump pyflakes from 2.2.0 to 2.3.0, flake8 from 3.8.4 to 3.9.0

Bumps [pyflakes](https://github.com/PyCQA/pyflakes) from 2.2.0 to 2.3.0.
- [Release notes](https://github.com/PyCQA/pyflakes/releases)
- [Changelog](https://github.com/PyCQA/pyflakes/blob/master/NEWS.rst)
- [Commits](https://github.com/PyCQA/pyflakes/compare/2.2.0...2.3.0)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

requirements-tests.txt

 -r requirements.txt
 pytest==6.2.2
 pytest-cov==2.11.1
-flake8==3.8.4
-pyflakes==2.2.0
+flake8==3.9.0
+pyflakes==2.3.0