[3.12] gh-75988: Fix issues with autospec ignoring wrapped object (GH-115223) (#117119)

gh-75988: Fix issues with autospec ignoring wrapped object (#115223)

* set default return value of functional types as _mock_return_value

* added test of wrapping child attributes

* added backward compatibility with explicit return

* added docs on the order of precedence

* added test to check default return_value

(cherry picked from commit 735fc2cbbcf875c359021b5b2af7f4c29f4cf66d)