asyncore_pollchoose import refactor

[coffeedogs]

src/network/asyncore_pollchoose.py could benefit from changes like this to imports, however it would imply importing errno directly which would require some refactoring of the file which is out of scope for the code_quality work which is already approved. This change was recommended by @g1itch

This issue is to ensure such refactoring is considered at some point.

[g1itch]

You can also use importlib to fool the lints, like here.

[PeterSurda]

This is based on code which is originally included in python 2.7, someone patched it to add epoll and kqueue support and I made further changes.

I don't remember exactly whether I added winsock handling fully, or just amended it based on my tests. I don't mind if it's changed, but it would need further testing and now's not the time for that.