Add test pass, classifiers for Python 3.9

[crwilcox]

Python 3.9 classifiers and tests should be added.

https://github.com/googleapis/python-storage/blob/master/setup.py#L84
https://github.com/googleapis/python-storage/blob/master/setup.py#L93
https://github.com/googleapis/python-storage/blob/master/noxfile.py#L30

[crwilcox]

Similarly, 3.5 should be dropped. it may make sense to drop 2.7 as well.