bpo-36728: Remove PyEval_ReInitThreads() from C API (GH-13241)

Remove the PyEval_ReInitThreads() function from the Python C API.
It should not be called explicitly: use PyOS_AfterFork_Child()
instead.

Rename PyEval_ReInitThreads() to _PyEval_ReInitThreads() and add a
'runtime' parameter.