Update flake8 from 3.7.5 to 3.7.6 (#10721)

requirements/codestyle_common.txt

@@ -1,9 +1,9 @@
 -r system.txt
 
 # flake8 is required by flake8-mutable
-flake8==3.7.5 \
+flake8==3.7.6 \
-    --hash=sha256:c3ba1e130c813191db95c431a18cb4d20a468e98af7a77e2181b68574481ad36 \
+    --hash=sha256:6d8c66a65635d46d54de59b027a1dda40abbe2275b3164b634835ac9c13fd048 \
-    --hash=sha256:fd9ddf503110bf3d8b1d270e8c673aab29ccb3dd6abf29bae1f54e5116ab4a91
+    --hash=sha256:6eab21c6e34df2c05416faa40d0c59963008fff29b6f0ccfe8fa28152ab3e383
 flake8-mutable==1.2.0 \
     --hash=sha256:ee9b77111b867d845177bbc289d87d541445ffcc6029a0c5c65865b42b18c6a6 \
     --hash=sha256:38fd9dadcbcda6550a916197bc40ed76908119dabb37fbcca30873666c31d2d5