[3.6] bpo-25532: Protect against infinite loops in inspect.unwrap() (GH-1717) (#3778)

[3.6] bpo-25532: Protect against infinite loops in inspect.unwrap() (GH-1717) (#3778)

Some objects (like test mocks) auto-generate new objects on
attribute access, which can lead to an infinite loop in
inspect.unwrap().

Ensuring references are retained to otherwise temporary objects
and capping the size of the memo dict turns this case into a
conventional exception instead..
(cherry picked from commit f9169ce6b48c7cc7cc62d9eb5e4ee1ac7066d14b)