Add .flake8 configuration

.flake8

@@ -0,0 +1,5 @@
+[flake8]
+# E203 is not PEP8 compliant and is not compatible with black:
+# https://black.readthedocs.io/en/stable/the_black_code_style.html#slices
+# The same goes of E231 and W503 conflicts with black, too.
+ignore = E203,E231,W503