bpo-17185: Add __signature__ to mock that can be used by inspect for signature (GH11125)

* Fix partial and partial method signatures in mock

* Add more calls

* Add NEWS entry

* Use assertEquals and fix markup in NEWS

* Refactor branching and add markup reference for functools

* Revert partial object related changes and fix pr comments
(cherry picked from commit f7fa62ef4422c9deee050a794fd8504640d9f8f4)

Co-authored-by: Xtreak <tirkarthi@users.noreply.github.com>
diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index f641e38..5907e5c 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -102,6 +102,7 @@
         sig.bind(*args, **kwargs)
     _copy_func_details(func, checksig)
     type(mock)._mock_check_sig = checksig
+    type(mock).__signature__ = sig
 
 
 def _copy_func_details(func, funcopy):
@@ -171,11 +172,11 @@
     return mock(*args, **kwargs)""" % name
     exec (src, context)
     funcopy = context[name]
-    _setup_func(funcopy, mock)
+    _setup_func(funcopy, mock, sig)
     return funcopy
 
 
-def _setup_func(funcopy, mock):
+def _setup_func(funcopy, mock, sig):
     funcopy.mock = mock
 
     # can't use isinstance with mocks
@@ -223,6 +224,7 @@
     funcopy.assert_called = assert_called
     funcopy.assert_not_called = assert_not_called
     funcopy.assert_called_once = assert_called_once
+    funcopy.__signature__ = sig
 
     mock._mock_delegate = funcopy