Use of IterableUserDict and basestring incompatible with Python 3

[SpotlightKid]

flask_zodb.py uses the UserDict.IterableUserDict class, which is replaced by collections.UserDict in Python 3.

Also, the ZODB.create_db method uses basestring, which is unavailable in Python.

This can be fixed easily with the following patch (I'll also make a pull request), making Flask-ZODB work with Python 3:

diff --git a/flask_zodb.py b/flask_zodb.py
index a950cf1..5c9f582 100644
--- a/flask_zodb.py
+++ b/flask_zodb.py
@@ -2,7 +2,11 @@ import flask
 import transaction
 import zodburi

-from UserDict import IterableUserDict
+try:
+    from collections import UserDict
+except ImportError: # for Python 2
+    from UserDict import IterableUserDict as UserDict
+
 from ZODB.DB import DB
 from contextlib import contextmanager, closing
 from werkzeug.utils import cached_property
@@ -12,11 +16,17 @@ from persistent import Persistent as Object
 from persistent.list import PersistentList as List
 from persistent.mapping import PersistentMapping as Dict

+# Python 3 compatibility
+try:
+    basestring
+except NameError:
+    basestring = str
+

 __all__ = ['ZODB', 'Object', 'List', 'Dict', 'BTree']


-class ZODB(IterableUserDict):
+class ZODB(UserDict):
     """Extension object.  Behaves as the root object of the storage during
     requests, i.e. a `~persistent.mapping.PersistentMapping`.