Try to use collections.abc.Mapping instead of collections.Mapping

The latter is deprecated and will be removed in Python 3.9, fixes #76

parso/python/tree.py

@@ -43,7 +43,10 @@ Parser Tree Classes
 """
 
 import re
-from collections import Mapping
+try:
+    from collections.abc import Mapping
+except ImportError:
+    from collections import Mapping
 
 from parso._compatibility import utf8_repr, unicode
 from parso.tree import Node, BaseNode, Leaf, ErrorNode, ErrorLeaf, \