python/sepolgen: return NotImplemented instead of raising it

sepolgen uses "return NotImplemented" (in access.py and matching.py) in
order to make Python's sorting function use an other call to compare
objects. For this to work, "NotImplemented" needs to be returned, not
raised like _compare's default implementation does.

This issue has been found using flake8. This Python linter reported:

    python/sepolgen/src/sepolgen/util.py:128:9: F901 'raise
    NotImplemented' should be 'raise NotImplementedError'

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

python/sepolgen/src/sepolgen/util.py

@@ -123,7 +123,7 @@ class Comparison():
     _compare function within your class."""
 
     def _compare(self, other, method):
-        raise NotImplemented
+        return NotImplemented
 
     def __eq__(self, other):
         return self._compare(other, lambda a, b: a == b)