chore: python 3.9 as the default python version for noxfile.py

[parthea]

#1069 was merged which upgrades the python version in the post processor image from python 3.8 to 3.9. The default version for noxfile.py should also be updated.

[parthea]

@busunkim96 Please could you review/approve?

[busunkim96]

I'm curious if this is worth the trouble at this point in time? I know you've moved many repos to the templated noxfile but this will cause issues for any noxfiles that aren't - the noxfile will want to use 3.8 for nox -s blacken while the owlbot postprocessor will only have 3.9.

Perhaps it would be better to move the blackening outside of owlbot.py, so changing this version is not as much of a chore.

[parthea]

Superseded by #1144 .

I've added an agenda item to the python client libraries meeting to discuss moving the blacken session outside of owlbot.py