Bump flake8 from 3.9.0 to 3.9.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.9.0 to 3.9.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.9.0...3.9.1)

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

test-requirements.txt

@@ -94,9 +94,9 @@ coveralls==3.0.1 \
 docopt==0.6.2 \
     --hash=sha256:49b3a825280bd66b3aa83585ef59c4a8c82f2c8a522dbe754a8bc8d08c85c491
     # via coveralls
-flake8==3.9.0 \
-    --hash=sha256:12d05ab02614b6aee8df7c36b97d1a3b2372761222b19b58621355e82acddcff \
-    --hash=sha256:78873e372b12b093da7b5e5ed302e8ad9e988b38b063b61ad937f26ca58fc5f0
+flake8==3.9.1 \
+    --hash=sha256:1aa8990be1e689d96c745c5682b687ea49f2e05a443aff1f8251092b0014e378 \
+    --hash=sha256:3b9f848952dddccf635be78098ca75010f073bfe14d2c6bda867154bea728d2a
     # via -r test-requirements.in
 idna==2.10 \
     --hash=sha256:b307872f855b18632ce0c21c5e45be78c0ea7ae4c15c828c20788b26921eb3f6 \
@@ -105,7 +105,11 @@ idna==2.10 \
 importlib-metadata==3.10.0 \
     --hash=sha256:c9db46394197244adf2f0b08ec5bc3cf16757e9590b02af1fca085c16c0d600a \
     --hash=sha256:d2d46ef77ffc85cbf7dac7e81dd663fde71c45326131bea8033b9bad42268ebe
-    # via -r test-requirements.in
+    # via
+    #   -r test-requirements.in
+    #   flake8
+    #   pluggy
+    #   pytest
 iniconfig==1.1.1 \
     --hash=sha256:011e24c64b7f47f6ebd835bb12a743f2fbe9a26d4cecaa7f53bc4f35ee9da8b3 \
     --hash=sha256:bc3af051d7d14b2ee5ef9969666def0cd1a000e121eaea580d4a313df4b37f32
@@ -310,6 +314,7 @@ typed-ast==1.4.3 \
     --hash=sha256:f8afcf15cc511ada719a88e013cec87c11aff7b91f019295eb4530f96fe5ef2f \
     --hash=sha256:fb1bbeac803adea29cedd70781399c99138358c26d05fcbd23c13016b7f5ec65
     # via
+    #   astroid
     #   black
     #   mypy
 typing-extensions==3.7.4.3 \
@@ -318,6 +323,7 @@ typing-extensions==3.7.4.3 \
     --hash=sha256:dafc7639cde7f1b6e1acc0f457842a83e722ccca8eef5270af2d74792619a89f
     # via
     #   black
+    #   importlib-metadata
     #   mypy
 urllib3==1.26.4 \
     --hash=sha256:2f4da4594db7e1e110a944bb1b551fdf4e6c136ad42e4234131391e21eb5b0df \