Bump flake8 from 3.8.4 to 3.9.0

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)

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

test-requirements.txt

@@ -33,7 +33,7 @@ cryptography==3.4.6
     #   trustme
 decorator==4.4.2
     # via ipython
-flake8==3.8.4
+flake8==3.9.0
     # via -r test-requirements.in
 idna==2.10
     # via
@@ -86,11 +86,11 @@ ptyprocess==0.7.0
     # via pexpect
 py==1.10.0
     # via pytest
-pycodestyle==2.6.0
+pycodestyle==2.7.0
     # via flake8
 pycparser==2.20
     # via cffi
-pyflakes==2.2.0
+pyflakes==2.3.0
     # via flake8
 pygments==2.7.4
     # via ipython