bpo-39264: Fix UserDict.get() to account for __missing__() (GH-17910)

Here's the patch according to the discussion at the [Python-Dev mailing list](https://mail.python.org/archives/list/python-dev@python.org/thread/SDXOEMAEM6KQ3CQCJVBVRT5QNSPAVU6X/).
UserDict.get() will match dict's behavior and not call `__missing__`.

Automerge-Triggered-By: GH:rhettinger