allow multiprocess dep instead of multiprocessing

[lalo]

multiprocess external lib has other benefits like using dill instead of pickle, allowing us more leeway on certain edge cases that are not compatible with native multiprocessing.

https://github.com/uqfoundation/multiprocess

from their readme:

multiprocess enables:

objects to be transferred between processes using pipes or multi-producer/multi-consumer queues
objects to be shared between processes using a server process or (for simple data) shared memory

multiprocess provides:

equivalents of all the synchronization primitives in threading
a Pool class to facilitate submitting tasks to worker processes
enhanced serialization, using dill

Let me know your thoughts on this type of change. Happy to iterate on it.

Thanks

Related: #53

[cgarciae]

I wonder why CI didn't trigger.

[cgarciae]

Hey @lalo, sorry for being out so long.
I like the change but I'd prefer if this was under a configuration flag, it could also be generalized to other implementations e.g:

from pypeln import config
import multiprocess

config.set_multiprocessing_impl(multiprocess)

[lalo]

I'll take a look

[lalo]

this inheritance of Queue might complicate things

[cgarciae]

Typehints should stay the same, the important thing would be to use a different module when initializing Queue and Process at runtime.

[lalo]

heres a wip impl #100