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.

scripts/synapse_port_db

@@ -501,7 +501,8 @@ class Porter(object):
 
             try:
                 yield self.postgres_store.runInteraction("alter_table", alter_table)
-            except Exception as e:
+            except Exception:
+                # On Error Resume Next
                 pass
 
             yield self.postgres_store.runInteraction(