Update flake8 from 3.7.8 to 3.7.9 (#12743)

requirements/codestyle.txt

@@ -1,9 +1,9 @@
 -r system.txt
 
 # flake8 is required by flake8-mutable
-flake8==3.7.8 \
-    --hash=sha256:19241c1cbc971b9962473e4438a2ca19749a7dd002dd1a946eaba171b4114548 \
-    --hash=sha256:8e9dfa3cecb2400b3738a42c54c3043e821682b9c840b0448c0503f781130696
+flake8==3.7.9 \
+    --hash=sha256:45681a117ecc81e870cbf1262835ae4af5e7a8b08e40b944a8a6e6b895914cfb \
+    --hash=sha256:49356e766643ad15072a789a20915d3c91dc89fd313ccd71802303fd67e4deca
 flake8-mutable==1.2.0 \
     --hash=sha256:ee9b77111b867d845177bbc289d87d541445ffcc6029a0c5c65865b42b18c6a6 \
     --hash=sha256:38fd9dadcbcda6550a916197bc40ed76908119dabb37fbcca30873666c31d2d5