Bug 1700680 [wpt PR 28217] - Update pyflakes to 2.3.1, a=testonly

Automatic update from web-platform-tests
Update pyflakes from 2.3.0 to 2.3.1
--

wpt-commits: fddab4d2ee0b6387e55c83cf3f69083de2bd118a
wpt-pr: 28217

testing/web-platform/tests/tools/requirements_flake8.txt

@@ -1,4 +1,4 @@
 flake8==3.9.0
 pycodestyle==2.7.0
-pyflakes==2.3.0
+pyflakes==2.3.1
 pep8-naming==0.11.1