setup.cfg [flake8]: Use "extend-ignore" instead of "ignore"

That way, we do not have to hardcode the list of rules that flake8
ignores by default just so we could add our own list of extra ignores.
This also brings the ignore list in line with what AdaCore's style_check
does.

Change-Id: I717e2918dd23a46440e96b2319129cddd23df8f9

setup.cfg

@@ -1,8 +1,4 @@
 [flake8]
-# Ignore list:
-# D100,D101,D102,D102,D103,D104,D105,D106,D107: Missing docstring
-# W503: line-break before binary operator (we have to ignore either
-#       this one, or W504, which is about line-break *after* binary
-#       operator).
-ignore = D100,D101,D102,D102,D103,D104,D105,D106,D107,W503
+# Follow what the style_checker does in terms of the extend-ignore...
+extend-ignore = E203,E402
 exclude = tests