github/workflows: Use Python 3.8 for macos workflow.
github/workflows: Use Python 3.8 for macos workflow.

Otherwise it gets a more recent version which fails some of the test suite.

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython