supervisor: migrate to python@3.9

[fxcoudert]

As part of the Python 3.9 migration (#62201).

This formula is independent from the all other Python formulas (if I didn't screw up my script or my logic)

Do not merge before the next Brew tag ships, expected on Monday 2020-10-12

[fxcoudert]

==> /usr/local/Cellar/supervisor/4.2.0_1/bin/supervisorctl -c sd.ini version
unix:///private/tmp/supervisor-test-20201007-47758-7n7urp/supervisor.sock no such file
Error: supervisor: failed
An exception occurred within a child process:
  Test::Unit::AssertionFailedError: <0> expected but was
<7>.

[BrewTestBot]

@fxcoudert has triggered a merge.

[BrewTestBot]

⚠️ @fxcoudert bottle publish failed.

[BrewTestBot]

@fxcoudert has triggered a merge.