Fix a number of flake8 errors

Broadly three things here:

* disable W504 which seems a bit whacko
* remove a bunch of `as e` expressions from exception handlers that don't use
  them
* use `r""` for strings which include backslashes

Also, we don't use pep8 any more, so we can get rid of the duplicate config
there.

synapse/federation/federation_server.py

@@ -800,7 +800,7 @@ class FederationHandlerRegistry(object):
             yield handler(origin, content)
         except SynapseError as e:
             logger.info("Failed to handle edu %r: %r", edu_type, e)
-        except Exception as e:
+        except Exception:
             logger.exception("Failed to handle edu %r", edu_type)
 
     def on_query(self, query_type, args):