Explicitly add flake8 selections

changelog.d/9370.misc

@@ -0,0 +1 @@
+Explicitly select flake8 checks (as opposed to the defaults).

setup.cfg

@@ -10,14 +10,36 @@ ignore =
  tox.ini
 
 [flake8]
-# see https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
-# for error codes. The ones we ignore are:
-# W503: line break before binary operator
-# W504: line break after binary operator
-# E203: whitespace before ':' (which is contrary to pep8?)
-# E731: do not assign a lambda expression, use a def
-# E501: Line too long (black enforces this for us)
-ignore=W503,W504,E203,E731,E501
+select=
+ # Errors and Warnings
+ # https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
+ E, W
+ # pyflakes
+ # https://flake8.pycqa.org/en/latest/user/error-codes.html
+ F
+ # mccabe
+ # https://github.com/pycqa/mccabe#plugin-for-flake8
+ # (not enabled unless max-complexity is set)
+ C90
+ # flake8-comprehensions
+ # https://github.com/adamchainz/flake8-comprehensions#rules
+ C4
+
+# Ignored checks take precedence over selected checks.
+# Please also include a description as to why to include these, if possible.
+ignore=
+ # "line break before binary operator"
+ W503
+ # "line break after binary operator"
+ W504
+ # "Whitespace before ':'"
+ # (Used to circumvent colon-in-slice-index false-positives, see #9368)
+ E203
+ # "Do not assign a lambda expression, use a def"
+ E731
+ # "Line too long"
+ # (No need to check with flake8, black enforces this for us)
+ E501
 
 [isort]
 line_length = 88