bpo-38069: Convert _posixsubprocess to PEP-384

[DinoV]

Summary:
Eliminate uses of _Py_IDENTIFIER from _posixsubprocess, replacing them with interned strings.

Also tries to find an existing version of the module, which will allow subinterpreters.

https://bugs.python.org/issue38069

Automerge-Triggered-By: @tiran

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[gpshead]

Thanks for doing this modernization, i assume you're taking this on for a pile of stdlib modules? yay!

[serhiy-storchaka]

This looks as unneeded complication to me. What is the benefit of all these changes? Why get rid of _Py_IDENTIFIER? Why _posixsubprocessstate_global->enable is better than &PyId_enable?

[tiran]

The current _Py_IDENTIFIER approach is not compatible with sub-interpreters. In the long run the identifier has to be replaced with a different implementation. The interned interpreter state strings is one good approach.