Include python 3.9 in stdlibs

[michaelhays]

isort isn't recognizing zoneinfo as a part of the standard library unless I specify --py=39.

I noticed that a stdlibs/py39.py file exists (with zoneinfo), but that it isn't included in stdlibs/py3.py. I didn't look too much into how it works, but seeing that the Python Version configuration docs says the "Default is to assume any Python 3 version could be the target, and use a union of all stdlib modules across versions", I'd assume it would just work if it were added to py3.py.

(And of course, thanks for all of your work on this excellent tool!)

[timothycrosley]

Thanks for reporting! With the just release isort 5.6.0 release python3.9 imports are defaulted to being detected as stdlib now that Python3.9 has also been released.

Thanks!

~Timothy