Add conditional import of MutableMapping

Importing the abstract base classes directly from the collections module
is deprecated in Python 3.7. They have been moved to collections.abc and
the deprecated way will stop working in Python 3.8.

See: https://docs.python.org/3/library/collections.html

Author: erikagnvall

symengine/lib/symengine_wrapper.pyx

@@ -19,6 +19,12 @@ import collections
 import warnings
 from symengine.compatibility import is_sequence
 import os
+import sys
+
+if sys.version_info[0] == 2:
+    from collections import MutableMapping
+else:
+    from collections.abc import MutableMapping
 
 try:
     import numpy as np
@@ -720,7 +726,7 @@ cdef class _DictBasic(object):
         return d
 
 
-class DictBasic(_DictBasic, collections.MutableMapping):
+class DictBasic(_DictBasic, MutableMapping):
 
     def __str__(self):
         return "{" + ", ".join(["%s: %s" % (str(key), str(value)) for key, value in self.items()]) + "}"