Backport fix from Python-3.7.

https://gerrit.chromium.org/gerrit/45771

--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -130,4 +138,7 @@ def _callable(obj):
     if isinstance(obj, type):
         return True
+    from types import MethodType
+    if isinstance(obj, (staticmethod, classmethod, MethodType)):
+        return _callable(obj.__func__)
     if getattr(obj, '__call__', None) is not None:
         return True