Merge changes from MicroPython v.122 release; annotate CIRCUITPY-CHANGES

[dhalbert]

I meant to merge this in long ago. These are changes from the merge of MicroPython v1.22 to CircuitPython. They were in the frozen version of asyncio, which was also used for testing. But they were not merged into the library, to avoid version skew issues at the time. Now they should be merged.

Changes:

• Incorporate upstream changes, including fixing state of cur_task and other things.
• Remove ThreadSafeFlag: it's in MicroPython, but not in CPython. If we need something like this, put it back later.
• Add get_running_loop()
• Use await instead of yield.
• Restore gather to be async.
• Make comments be more CircuitPython-centric.
• Add CIRCUITPY-CHANGE annotations to note code changes from MicroPython, to ease later merges.

I tested this merge against current CircuitPython, and it passes all the tests.

Because this makes incompatible changes to the API, it should be 2.0.0.

[tannewt]

Thank you!