[issue45743] Cleanup and simplify setup.py

[Erlend E. Aasland]

Erlend E. Aasland <erlend.aasland at innova.no> added the comment:

> 1) __APPLE_USE_RFC_3542 should have been in socketmodule.c from the start, not sure why it was added in setup.py.

FTR, it was added by me in bpo-35569, GH-19526. I moved the check to setup.py because Ned made me do it :P https://github.com/python/cpython/pull/19526#discussion_r415637170

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