Update flake8 to 3.9.2

This was necessary to mitigate an exception on Python 3.9:
AttributeError: 'FlakesChecker' object has no attribute 'CONSTANT'

Author: ckoehn

tox.ini

@@ -15,6 +15,6 @@ install_command=python -m pip install {env:TOX_PIP_OPTS:} {opts} {packages} -r t
 [testenv:lint]
 skip_install = True
 deps =
-  flake8==3.6.0
+  flake8==3.9.2
 commands =
   flake8 sqlalchemy_cockroachdb test