Import MutableMapping from collections.abc

Python 3.3 and above moved the abstract base classes to their own
module under collections, collections.abc. Import from that location,
falling back if required for Python 3.2.

Fixes blaze#29

chest/core.py

@@ -1,4 +1,7 @@
-from collections import MutableMapping
+try:
+    from collections.abc import MutableMapping
+except ImportError:  # pragma: no cover
+    from collections import MutableMapping  # pragma: no cover
 from functools import partial
 from threading import Lock
 from contextlib import contextmanager