Add a setup.cfg to make flake8 compatible with blackified files

Depends on !142 (closed).

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Migrated from D2954 (view on Phabricator)

setup.cfg

+[flake8]
+# E203: whitespaces before ':' <https://github.com/psf/black/issues/315>
+# E231: missing whitespace after ','
+# W503: line break before binary operator <https://github.com/psf/black/issues/52>
+ignore = E203,E231,W503
+max-line-length = 88