I wanted to see if it was possible to use the CPython standard library on the M4 so I did a test and it looks promising. The unittest module which is probably one of the toughest wrt. demands on CircuitPython features did work with a small set of changes.

Here's the port of the os, time and unittest modules: https://github.com/notro/tmp_CircuitPython_stdlib

These are the CircuitPython changes: stdlib.diff

The CP changes won't fit on the already overflowing boards, but for M4 it's a small change (1-2kB I think).

The next generation following the M4 will have even more RAM so I think this is the way to go, but the question is if this is the right time to do it.