Fix .flake8 for fake8 6.0.0

This backwards incompatible change was made in version 6.0.0 of
flake8.

* Produce an error when invalid codes are specified in configuration
  (See also #1689, #1713).

see: PyCQA/flake8#1689 (comment)

.flake8

@@ -1,9 +1,12 @@
 [flake8]
 doctests = True
 ignore =
-  E203, # whitespace before ‘:’
-  E501, # line too long
-  W503, # line break before binary operator
+  # whitespace before ‘:’
+  E203,
+  # line too long
+  E501,
+  # line break before binary operator
+  W503,
 exclude =
   .git,
   .mypy_cache,