maint: move .flake8 config into setup.cfg

Author: oesteban

.flake8

@@ -91,6 +91,8 @@ parentdir_prefix =
 max-line-length = 99
 doctests = True
 exclude=*build/
+select = C,E,F,W,B,B950
+ignore = N802,N806,W503,E203
 putty-ignore =
     */__init__.py : +F401
     docs/conf.py : +E265