Bump flake8 from 3.8.4 to 3.9.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.1)

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

setup.cfg

@@ -34,7 +34,7 @@ dev =
     Sphinx==3.4.3
     black==20.8b1
     bump2version==1.0.1
-    flake8==3.8.4
+    flake8==3.9.1
     flake8-absolute-import>=1.0
     flake8-implicit-str-concat==0.2.0
     flake8-print==4.0.0
@@ -49,7 +49,7 @@ doc =
     furo==2020.12.30b24
 lint =
     black==20.8b1
-    flake8==3.8.4
+    flake8==3.9.1
     flake8-absolute-import>=1.0
     flake8-implicit-str-concat==0.2.0
     flake8-print==4.0.0