bpo-39485: fix corner-case in method-detection of mock (GH-18255)

Replace check for whether something is a method in the mock module. The
previous version fails on PyPy, because there no method wrappers exist
(everything looks like a regular Python-defined function). Thus the
isinstance(getattr(result, '__get__', None), MethodWrapperTypes) check
returns True for any descriptor, not just methods.

This condition could also return erroneously True in CPython for
C-defined descriptors.

Instead to decide whether something is a method, just check directly
whether it's a function defined on the class. This passes all tests on
CPython and fixes the bug on PyPy.
(cherry picked from commit a327677905956ae0b239ff430a1346dfe265709e)

Co-authored-by: Carl Friedrich Bolz-Tereick <cfbolz@gmx.de>

Co-authored-by: Carl Friedrich Bolz-Tereick <cfbolz@gmx.de>
diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index 66ace80..204b3e7 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -2715,7 +2715,7 @@
             continue
         if isinstance(result, (staticmethod, classmethod)):
             return False
-        elif isinstance(getattr(result, '__get__', None), MethodWrapperTypes):
+        elif isinstance(result, FunctionTypes):
             # Normal method => skip if looked up on type
             # (if looked up on instance, self is already skipped)
             return is_type
@@ -2745,10 +2745,6 @@
     type(ANY.__eq__),
 )
 
-MethodWrapperTypes = (
-    type(ANY.__eq__.__get__),
-)
-
 
 file_spec = None