set_forkserver_preload should check type of elements in passed list

[mawi2017]

Checklist

• I am confident this is a bug in CPython, not a bug in a third-party project
• I have searched the CPython issue tracker,
  and am confident this bug has not been reported before

CPython versions tested on:

3.10

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.10.12 (main, Jul 5 2023, 18:54:27) [GCC 11.2.0]

A clear and concise description of the bug:

The code for set_forkserver_preload (also in the latest main branch) should check the type of the elements passed through the module_names parameter, but instead checks the elements of self._preload_modules.

Code for reproducing:

#!/usr/bin/env python3
import multiprocessing as mp
import time

def work(param):
    print("work")

if __name__ == '__main__':
    ctx = mp.get_context('forkserver')
    ctx.set_forkserver_preload(['time', 1])
    
    with ctx.Pool(2) as p:
        p.map(work, [0, 1, 2, 3 ])

Raises error:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../lib/python3.10/multiprocessing/forkserver.py", line 178, in main
    __import__(modname)
TypeError: __import__() argument 1 must be str, not int

Linked PRs

• gh-107963: Fix set_forkserver_preload to check the type of given list #107965
• [3.12] gh-107963: Fix set_forkserver_preload to check the type of given list (GH-107965) #107975
• [3.11] gh-107963: Fix set_forkserver_preload to check the type of given list (GH-107965) #107976