run_tests/release: require Python 3.9+

While we support running `repo` on clients with older Python versions,
we don't need to hold the runners & release code back.  These are only
used by repo devs on their systems to develop & release repo.

Python 3.9 was picked due to its typing changs which we've already
started using in this code.

Change-Id: I6f8885c84298760514c25abeb1fccb0338947bf4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/539801
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

release/sign-tag.py

@@ -30,6 +30,9 @@ import sys
 import util
 
 
+assert sys.version_info >= (3, 9), "Release framework requires Python 3.9+"
+
+
 # We currently sign with the old DSA key as it's been around the longest.
 # We should transition to RSA by Jun 2020, and ECC by Jun 2021.
 KEYID = util.KEYID_DSA