Avoid the multiprocessing forkserver method in Python 3.14

[befeleme]

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• Do tests and lints pass with this change?
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• Is the new or changed code fully tested?
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Description of change

This change is essentially a copy of: giampaolo/pyftpdlib#656

Python 3.14 has changed the default multiprocessing method from fork to forkserver which causes failures like this one:

____________________ TestMultiprocessing.test_redis_client _____________________

self = <tests.test_multiprocessing.TestMultiprocessing object at 0x7f646b43d250>
r = <redis.client.Redis(<redis.connection.ConnectionPool(<redis.connection.Connection(host=localhost,port=6379,db=0)>)>)>

    def test_redis_client(self, r):
        "A redis client created in a parent can also be used in a child"
        assert r.ping() is True
    
        def target(client):
            assert client.ping() is True
            del client
    
        proc = multiprocessing.Process(target=target, args=(r,))
>       proc.start()

tests/test_multiprocessing.py:163: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/lib64/python3.14/multiprocessing/process.py:121: in start
    self._popen = self._Popen(self)
/usr/lib64/python3.14/multiprocessing/context.py:224: in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
/usr/lib64/python3.14/multiprocessing/context.py:300: in _Popen
    return Popen(process_obj)
/usr/lib64/python3.14/multiprocessing/popen_forkserver.py:35: in __init__
    super().__init__(process_obj)
/usr/lib64/python3.14/multiprocessing/popen_fork.py:20: in __init__
    self._launch(process_obj)
/usr/lib64/python3.14/multiprocessing/popen_forkserver.py:47: in _launch
    reduction.dump(process_obj, buf)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

obj = <Process name='Process-9' parent=996 initial>
file = <_io.BytesIO object at 0x7f646bbee390>, protocol = None

    def dump(obj, file, protocol=None):
        '''Replacement for pickle.dump() using ForkingPickler.'''
>       ForkingPickler(file, protocol).dump(obj)
E       _pickle.PicklingError: Can't pickle local object <function TestMultiprocessing.test_redis_client.<locals>.target at 0x7f646bb0f530>
E       when serializing dict item '_target'
E       when serializing multiprocessing.context.Process state
E       when serializing multiprocessing.context.Process object

I verified this makes tests pass with Python 3.14.0a2 in Fedora's build environment

[petyaslavova]

@befeleme please check linters failure

[befeleme]

Hopefully mitigated.

[befeleme]

One more amended change: ' -> "

[petyaslavova]

Hi @befeleme, the linters are failing. You can run them locally to check if there is something else than the one reported from the pipeline, the instructions are available in the CONTRIBUTING.md file