bpo-41006: importlib.util no longer imports typing

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Create importlib._abc submodule to avoid importing typing when
importlib.util is imported. Move Loader ABC into importlib._abc.

https://bugs.python.org/issue41006

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Another approach would be to move Traversable and TraversableReader classes to importlib.readers, but it seems like importlib.readers is not documented.

I chose to write a PR which doesn't affect the importlib API.

The from typing import Protocol, runtime_checkable line was added to Lib/importlib/abc.py by the commit 7f7e706 of bpo-39791.

cc @jaraco

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3 hours later, no Travis CI build was created. I close/reopen my PR to force Travis CI to schedule a new build.

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Thanks for the review @brettcannon!