Fixed py3 compat problem:

py3 b64decode will raise binascii.Error when the decoding failed.
To catch it, added new exception.

Author: hirokiky

oauth2_provider/oauth2_validators.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import base64
+import binascii
 import logging
 from datetime import timedelta
 
@@ -56,7 +57,7 @@ def _authenticate_basic_auth(self, request):
 
         try:
             b64_decoded = base64.b64decode(auth_string)
-        except TypeError:
+        except (TypeError, binascii.Error):
             log.debug("Failed basic auth: %s can't be decoded as base64", auth_string)
             return False