[3.6] bpo-20891: Py_Initialize() now creates the GIL (#4700)

[vstinner]

The GIL is no longer created "on demand" to fix a race condition when
PyGILState_Ensure() is called in a non-Python thread.

Reenable test_capi.test_bpo20891().

(cherry picked from commit 2914bb3)

https://bugs.python.org/issue20891

[vstinner]

@pitrou: Do you think that it's ok to make such change in the stable 3.6? IMHO it's a bugfix and so must be backported to 3.6.

[pitrou]

@vstinner I don't think so. People can already call PyEval_InitThreads.

[vstinner]

@vstinner I don't think so. People can already call PyEval_InitThreads.

Since only two users complained about https://bugs.python.org/issue20891 in 3 years, I agree that it's ok to not fix Python 2.7 and 3.6. The workaround is to call PyEval_InitThreads() before starting the first thread.

I wasn't excited to make such stable in stable 2.7 and 3.6 anyway :-)