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/rest/client/v2_alpha/auth.py

@@ -99,7 +99,7 @@ class AuthRestServlet(RestServlet):
     cannot be handled in the normal flow (with requests to the same endpoint).
     Current use is for web fallback auth.
     """
-    PATTERNS = client_v2_patterns("/auth/(?P<stagetype>[\w\.]*)/fallback/web")
+    PATTERNS = client_v2_patterns(r"/auth/(?P<stagetype>[\w\.]*)/fallback/web")
 
     def __init__(self, hs):
         super(AuthRestServlet, self).__init__()

synapse/rest/media/v1/preview_url_resource.py

@@ -674,7 +674,7 @@ def summarize_paragraphs(text_nodes, min_size=200, max_size=500):
         # This splits the paragraph into words, but keeping the
         # (preceeding) whitespace intact so we can easily concat
         # words back together.
-        for match in re.finditer("\s*\S+", description):
+        for match in re.finditer(r"\s*\S+", description):
             word = match.group()
 
             # Keep adding words while the total length is less than