Reduce audio stuttering by running background tasks

[jepler]

This improves the situation where background tasks are starved by

• bulk I/O (.read())
• listdir
• printing

Testing performed: Using nRF I2S audio (not yet in master) I started a 22050Hz, mono, 16-bit sample running in a loop and then ran the reproducers at the REPL:
while True: os.listdir('.')
while True: len(open('somefile.wav').read())

This reduced the amount of buffering in I2S audio required to not hear stuttering from ~200ms to 16m, as reflected in the current state of #2127.

This may be related to #470, or then again it may not.

[tannewt]

Interesting! Please take a look at the test failure and fix it. Thanks!

[jepler]

@tannewt weird failures with the next CI run ... more rate limits being hit?

##[warning]Failed to download action 'https://api.github.com/repos/actions/setup-python/tarball/v1'. Error Response status code does not indicate success: 401 (Unauthorized).
##[warning]Back off 28.517 seconds before retry.
##[error]Response status code does not indicate success: 401 (Unauthorized).

[tannewt]

No, idea. I filed an issue about it: actions/setup-python#21 Ping there if you see it again.

I tried to re-run the checks but it might need a newer merge to test due to actions/checkout#23

[tannewt]

Thank you!