[3.11] gh-102978: Fix mock.patch function signatures for class and staticmethod decorators (GH-103228) (#103499)

Fixes unittest.mock.patch not enforcing function signatures for methods
decorated with @classmethod or @staticmethod when patch is called with
autospec=True.

(cherry picked from commit 59e0de4903)

Co-authored-by: Tomas R <tomas.roun8@gmail.com>

Lib/unittest/mock.py

@@ -98,6 +98,12 @@ def _get_signature_object(func, as_instance, eat_self):
         func = func.__init__
         # Skip the `self` argument in __init__
         eat_self = True
+    elif isinstance(func, (classmethod, staticmethod)):
+        if isinstance(func, classmethod):
+            # Skip the `cls` argument of a class method
+            eat_self = True
+        # Use the original decorated method to extract the correct function signature
+        func = func.__func__
     elif not isinstance(func, FunctionTypes):
         # If we really want to model an instance of the passed type,
         # __call__ should be looked up, not __init__.