Tell flake8 to ignore files that should not be checked

The Python files under build/ are mere copies, not worth checking.

_LinkChecker_configdata.py is a generated file, not stored in the VCS,
not really part of the source.

setup.cfg

@@ -22,6 +22,9 @@ ignore = *.rej
 filename =
     *.py
     ./linkchecker
+extend-exclude =
+    build/
+    _LinkChecker_configdata.py
 builtins =
     _
     _n