Update crypto.py

MutableMapping needs to be explicitly imported from collections.abc since 3.8
https://pyquestions.com/cannot-import-name-mutablemapping-from-collections

Author: mrvanes

src/xmlsec/crypto.py

@@ -4,6 +4,7 @@
 import logging
 import threading
 import six
+import sys
 from six.moves import xrange
 from xmlsec import constants
 from binascii import hexlify
@@ -21,6 +22,8 @@
 
 if six.PY2:
     from UserDict import DictMixin
+elif sys.version_info[:2] >= (3, 8):
+    from collections.abc import MutableMapping as DictMixin
 else:
     from collections.abc import MutableMapping as DictMixin