implement PEP 554 for use in test suite

[ericsnowcurrently]

Issue #22 is about making use of subinterpreters (specifically the PEP 554 API) in the CPython test suite.
Until PEP 554 is accepted (or if it isn't) we should add a PEP 554 implementation as Lib/test/support/_interpreters.py (e.g. test.support._interpreters).

[ericsnowcurrently]

@nanjekyejoannah, what's the status on this? IIRC you were blocked on a few other issues. I'm hoping we can get them resolved in the next week or so and then get this merged. What do you think?

[nanjekyejoannah]

@ericsnowcurrently, yes I have most of it working. I will need to take a day or two for final touches. I have exams this coming week till around 24th but I will be able to get the remaining work through after 24th. So we are talking about practically hearing from me by 28th. Is this fine?

[ericsnowcurrently]

Sounds good. Good luck on your exams! 😁

[nanjekyejoannah]

Blocked by this PR. I think @LewisGaul has looked at this longer. It makes sense for you to finish. @LewisGaul happy to know where to help.