Expose `Barrier` in the `multiprocessing.synchronize` module.

[YvesDup]

Bug report

Bug description:

Maybe I miss something, but the Barrier class is missing in the __all__ variable.

cpython/Lib/multiprocessing/synchronize.py

Lines 10 to 12 in 02d02f4

	__all__ = [
	'Lock', 'RLock', 'Semaphore', 'BoundedSemaphore', 'Condition', 'Event'
	]

I suggest to add it.

CPython versions tested on:

3.13, 3.14, 3.15, CPython main branch

Operating systems tested on:

No response