Remove _Py_InitializeMain() private function

[vstinner]

PEP 587 "Python Initialization Configuration" added a "Multi-Phase Initialization Private Provisional API":

• PyConfig._init_main
• _Py_InitializeMain()

The plan was to make the "core phase" more usable. But since Python 3.8, Python doesn't use this "Multi-Phase Initialization" API and I failed to find any user in 3rd party code.

I propose to remove this API in Python 3.14 without any deprecation since the feature was provisional and it's a private API.

Provisional:

A provisional API is one which has been deliberately excluded from the standard library’s backwards compatibility guarantees.

Linked PRs

• gh-129033: Remove _Py_InitializeMain() function #129034
• gh-129033: Remove _PyInterpreterState_SetConfig() function #129048

[vstinner]

cc @ncoghlan @ericsnowcurrently

[vstinner]

_PyInterpreterState_SetConfig() and _PyInterpreterState_GetConfigCopy() can also be removed if _Py_InitializeMain() is removed. PEP 741 "Python Configuration C API" added a public API for that: PyConfig_Set().

[ncoghlan]

Makes sense to me. If an embedding application wants to split up interpreter initialisation and main module execution they can:

1. Pass an empty string in run_command during initialisation
2. Execute the relevant code in __main__.__dict__ later