Bump flake8 from 3.8.3 to 3.8.4 (#692)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.3 to 3.8.4.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.3...3.8.4)

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: Pomax <pomax@nihongoresources.com>

dev-requirements.txt

@@ -6,7 +6,7 @@
 #
 autoflake==1.4            # via -r dev-requirements.in
 docutils==0.15.2          # via -c requirements.txt, -r dev-requirements.in
-flake8==3.8.3             # via -r dev-requirements.in
+flake8==3.8.4             # via -r dev-requirements.in
 importlib-metadata==1.6.0  # via flake8
 mccabe==0.6.1             # via flake8
 pycodestyle==2.6.0        # via flake8