Use six.moves.filter when filtering out from MXID

Python 2's filter() function and Python 3's don't return the same type when processing a string (respectively str and filter), therefore use six's compatibility mapping (which resolves to itertools.ifilter() if using Python2), then generate a string from the filtered list, in order to ensure consistent behaviour between Python 2 and Python 3.

synapse/types.py

@@ -17,6 +17,7 @@ import string
 from collections import namedtuple
 
 import attr
+from six.moves import filter
 
 from synapse.api.errors import SynapseError
 
@@ -240,7 +241,8 @@ def strip_invalid_mxid_characters(localpart):
     Returns:
         localpart (basestring): the localpart having been stripped
     """
-    return filter(lambda c: c in mxid_localpart_allowed_characters, localpart)
+    filtered = filter(lambda c: c in mxid_localpart_allowed_characters, localpart)
+    return "".join(list(filtered))
 
 
 UPPER_CASE_PATTERN = re.compile(b"[A-Z_]")