List channel interpreters

[LewisGaul]

WIP for ericsnowcurrently/multi-core-python#52

[nanjekyejoannah]

I think you are editing the right files. Can you add a test in Lib/test/test_xxsubinterpreters.py to test the low level function.

Also, I suggest that you open a PR on CPython for this low-level functionality. I would love to see If other core developers review it before I pull it for use in the high-level PEP 554 implementation. Just exclude the changes to the high level interpreters module as this is PEP 554 related work and not yet upstream. @ericsnowcurrently what do you think?

[LewisGaul]

Thanks for taking a look Joannah.

I'm happy to have a go at writing some tests - there's also some finishing off needed in the implementation, I was just wanting to check the implementation is along the right lines (i.e. providing a channel_list_interpreters(chan_id, send_or_receive) function to be called from the channel classes in interpreters.py).

I'll tidy up the diff (drop the PEP-554 stuff) and make a PR to upstream cpython when it's all been finalised - at this point I just wanted to check it's 'roughly correct' before spending more time on it!