Make the Bazel CI workflow test different Python versions.

Change-Id: I84b078f47e3754c0a10bc58a54cba3f3987bbb80
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63330
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>