Add Python 3.9 tests

[busunkim96]

Choose a few libraries (mix of GAPIC, handwrittens) to add 3.9 tests before the 3.9.0 release is made (release schedule).

Steps:

• Add 3.9 to the docker image.
• Add 3.9 unit/system tests to a handful of libraries. The extra version can be added via templates (see example).

CC @plamut

[meredithslota]

We are way past 3.9 — 3.11 was added recently. Closing as "fixed" and also "obsolete". :) #10812