add flake config that mirrors pre-commit config

We cannot configure pre-commit to use the config file, so
we have to replicate the config explicitly. See:
    pre-commit/pre-commit-hooks#112
    https://gitlab.com/pycqa/flake8/issues/10

Author: jbweston

.flake8

@@ -0,0 +1,5 @@
+[flake8]
+max-line-length = 500
+ignore = E203,E266,E501,W503
+max-complexity = 18
+select = B,C,E,F,W,T4,B9