bpo-37088: Add the sys.addpendingcall() function.

[1st1]

https://bugs.python.org/issue37088

[asvetlov]

I like it but we need more reviewers I guess

[asvetlov]

@vstinner what do you think?

[asvetlov]

The more I think the more I like it.
addpendingcall() is obviously the very low-level API but it can help asyncio a lot for initialization event loop policies and SIGCHLD handling on Linux.
Python allows subscribing on a signal from the main thread only.
asyncio has on-demand initialization strategy for loops and policies.
So, if I want to use asyncio from sub-thread only and want to execute subprocesses a way to execute some code in main thread if I'm now in sub-thread context the feature is pretty cool.

[csabella]

This looked like it was ready to be merged, maybe pending some comments from the bug tracker. @1st1, what do you think?

[asvetlov]

@vstinner had objections, perhaps this PR should be rejected.

[vstinner]

There is not clear rationale to justify the additon of the function, so I reject the feature. If you are still interesting by the function, please elaborate the rationale in https://bugs.python.org/issue37088