[stdlib] update multiprocessing.managers.BaseListProxy, multiprocessing.managers.list

[guoci]

1. update multiprocessing.managers.list's signature to follow builtins.list.__init__'s signature
2. add type stub for multiprocessing.get_context.copy
   src: https://github.com/python/cpython/blob/c41fce08a5d7aae8fd9e3fa25e0ba521600e097c/Lib/multiprocessing/managers.py#L1164-L1169
   CPython PR: https://github.com/python/cpython/pull/103133/files

[guoci]

Mypy stubtest is using inspect.signature to get the signature of multiprocessing.managers.BaseListProxy.copy, but the signature is incorrect, and that triggers the CI failure.

>>> import multiprocessing, inspect
>>> mp_context = multiprocessing.get_context('spawn')
>>> manager = mp_context.Manager()
>>> l = manager.list([1,2])
>>> l.copy(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    l.copy(None)
    ~~~~~~^^^^^^
  File "<string>", line 2, in copy
  File ".../python3.14/multiprocessing/managers.py", line 848, in _callmethod
    raise convert_to_error(kind, result)
TypeError: list.copy() takes no arguments (1 given)
>>> inspect.signature(l.copy) # INCORRECT signature
<Signature (*args, **kwds)>
>>> inspect.signature([].copy)
<Signature ()>

[srittau]

Mypy stubtest is using inspect.signature to get the signature of multiprocessing.managers.BaseListProxy.copy, but the signature is incorrect, and that triggers the CI failure.

You can just add the problematic signature to the appropriate allowlist in stdlib/@tests/stubtest_allowlists with a comment explaining the situation.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Thanks!

For future reference, please don't force push. We squash merge anyway, but force pushing makes reviewing the changes since the review harder. (Not much of an issue in this case, just a general note.)