Mocks created by mocker.patch.multiple are not handled like other patches #638

mocker.patch.multiple() returns a dict of the mocks it created, and pytest-mock only registers that dict. #629 made resetall() reset the mocks inside it, but two other features still treat the dict as the mock:

1. No context-manager warning. The PytestMockWarning hook is only installed on the dict (where it does nothing), not on the mocks:

   def test_multiple_warning(mocker):
       mocks = mocker.patch.multiple(Target, a=mocker.DEFAULT)
       with pytest.warns(PytestMockWarning):  # fails: DID NOT WARN
           with mocks["a"]:
               pass

   with mocker.patch.object(Target, "a"): does warn.

2. mocker.stop() can't stop a single mock.

   def test_multiple_stop_one(mocker):
       mocks = mocker.patch.multiple(Target, a=mocker.DEFAULT, b=mocker.DEFAULT)
       mocker.stop(mocks["a"])  # ValueError: This mock object is not registered

   Only mocker.stop(mocks) works, which undoes all of them. Supporting single mocks would need each attribute's patcher to be stopped on its own.

Both reproduced on 3.16.1 (Target is any class with attributes a and b).