Add pyup ignore entries for latest pyflakes/pycodestyle (#4098)

Since we can't update to them until there is a newer release of
flake8 that supports the newer versions.

Closes #3425.
Closes #3565.

requirements/dev.txt

@@ -89,10 +89,10 @@ mccabe==0.6.1 \
     --hash=sha256:dd8d182285a0fe56bace7f45b5e7d1a6ebcbf524e8f3bd87eb0f125271b8831f
 pycodestyle==2.3.1 \
     --hash=sha256:6c4245ade1edfad79c3446fadfc96b0de2759662dc29d07d80a6f27ad1ca6ba9 \
-    --hash=sha256:682256a5b318149ca0d2a9185d365d8864a768a28db66a84a2ea946bcc426766
+    --hash=sha256:682256a5b318149ca0d2a9185d365d8864a768a28db66a84a2ea946bcc426766  # pyup: <3 # Blocked on new release of flake8
 pyflakes==1.6.0 \
     --hash=sha256:08bd6a50edf8cffa9fa09a463063c425ecaaf10d1eb0335a7e8b1401aef89e6f \
-    --hash=sha256:8d616a382f243dbf19b54743f280b80198be0bca3a5396f1d2e1fca6223e8805
+    --hash=sha256:8d616a382f243dbf19b54743f280b80198be0bca3a5396f1d2e1fca6223e8805  # pyup: <2 # Blocked on new release of flake8
 
 pytest-django==3.4.3 \
     --hash=sha256:49e9ffc856bc6a1bec1c26c5c7b7213dff7cc8bc6b64d624c4d143d04aff0bcf \