Bump flake8 from 3.8.4 to 3.9.0 (#33)

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[bot] <support@github.com>

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

setup.py

@@ -25,7 +25,7 @@
 ]
 
 LINT_EXTRAS = [
-    'flake8==3.8.4',
+    'flake8==3.9.0',
     'flake8-print==4.0.0',
     'flake8-implicit-str-concat==0.2.0',
     'isort==5.7.0',