Fix test execution under Python 3.14

[SuperSandro2000]

see the Python 3.14 changelog for the breaking change to multiprocessing fork on linux per default
https://docs.python.org/3/whatsnew/3.14.html#:~:text=On%20Unix%20platforms%20other%20than%20macOS%2C%20%E2%80%98forkserver%E2%80%99%20is%20now%20the%20default%20start%20method%20for%20ProcessPoolExecutor%20(replacing%20%E2%80%98fork%E2%80%99).%20This%20change%20does%20not%20affect%20Windows%20or%20macOS%2C%20where%20%E2%80%98spawn%E2%80%99%20remains%20the%20default%20start%20method

Multiple tests failed with similar errors like:

_____________________________ test_peer_connection _____________________________ [gw0] linux -- Python 3.14.3 /nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/bin/python3.14

test_server_sync = [<socket.socket fd=41, family=2, type=1, proto=0, laddr=('127.0.0.1', 8366), raddr=('127.0.0.1', 35672)>] test_client_sync = <socket.socket fd=33, family=2, type=1, proto=0, laddr=('127.0.0.1', 35672), raddr=('127.0.0.1', 8366)> event_loop = <_UnixSelectorEventLoop running=False closed=False debug=False>

    def test_peer_connection(
        test_server_sync: list[socket.socket],
        test_client_sync: socket.socket,
        event_loop: asyncio.AbstractEventLoop,
    ) -> None:
        """Run a full flow of with a peer."""
        worker = ServerWorker(FERNET_TOKENS)
        valid = datetime.now(tz=UTC) + timedelta(days=1)
        aes_key = os.urandom(32)
        aes_iv = os.urandom(16)
        hostname = "localhost"
        fernet_token = create_peer_config(valid.timestamp(), hostname, aes_key, aes_iv)

>       worker.start()

tests/server/test_worker.py:43:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ /nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/lib/python3.14/multiprocessing/process.py:121: in start
    self._popen = self._Popen(self)
                  ^^^^^^^^^^^^^^^^^
/nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/lib/python3.14/multiprocessing/context.py:224: in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/lib/python3.14/multiprocessing/context.py:300: in _Popen
    return Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^
/nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/lib/python3.14/multiprocessing/popen_forkserver.py:35: in __init__
    super().__init__(process_obj)
/nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/lib/python3.14/multiprocessing/popen_fork.py:20: in __init__
    self._launch(process_obj)
/nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/lib/python3.14/multiprocessing/popen_forkserver.py:47: in _launch
    reduction.dump(process_obj, buf)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

obj = <ServerWorker name='ServerWorker-21' parent=272 initial> file = <_io.BytesIO object at 0x7fffef185c10>, protocol = None

    def dump(obj, file, protocol=None):
        '''Replacement for pickle.dump() using ForkingPickler.'''
>       ForkingPickler(file, protocol).dump(obj)
E       TypeError: cannot pickle 'weakref.ReferenceType' object
E       when serializing dict item '_weakref'
E       when serializing multiprocessing.util.Finalize state
E       when serializing multiprocessing.util.Finalize object
E       when serializing dict item 'finalizer'
E       when serializing multiprocessing.popen_forkserver.Popen state
E       when serializing multiprocessing.popen_forkserver.Popen object
E       when serializing dict item '_popen'
E       when serializing multiprocessing.context.ForkServerProcess state
E       when serializing multiprocessing.context.ForkServerProcess object
E       when serializing dict item '_process'
E       when serializing multiprocessing.managers.SyncManager state
E       when serializing multiprocessing.managers.SyncManager object
E       when serializing dict item '_manager'
E       when serializing snitun.server.worker.ServerWorker state
E       when serializing snitun.server.worker.ServerWorker object

/nix/store/8gnchv834z56s561v3sx2h0ra1a2xn46-python3-3.14.3/lib/python3.14/multiprocessing/reduction.py:60: TypeError ---------------------------- Captured stderr setup ----------------------------- DEBUG:asyncio:Using selector: EpollSelector
------------------------------ Captured log setup ------------------------------
DEBUG    asyncio:selector_events.py:64 Using selector: EpollSelector