bpo-27448: Work around a gc.disable race condition in subprocess. (#1932)

* bpo-27448: Work around a gc.disable race condition in subprocess.

This works around a gc.isenabled/gc.disable race condition in the 2.7
subprocess module by using a lock for the critical section.  It'll
prevent multiple simultaneous subprocess launches from winding up with
gc remaining disabled but it can't fix the ultimate problem: gc enable
and disable is a global setting and a hack.

Users are *strongly encouraged* to use subprocess32 from PyPI instead
of the 2.7 standard library subprocess module.  Mixing threads with
subprocess is a recipie for disaster otherwise even with "fixes" to
ameliorate common issues like this.

* Add a blurb!

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