cleanup: Update codebase to expect Python 3.6

- Bump minimum version to Python 3.6.
- Use f-strings in a lot of places.

Change-Id: I2aa70197230fcec2eff8e7c8eb754f20c08075bb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389034
Tested-by: Jason R. Coombs <jaraco@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

subcmds/checkout.py

@@ -96,7 +96,7 @@ The command is equivalent to:
             all_projects,
             callback=_ProcessResults,
             output=Progress(
-                "Checkout %s" % (nb,), len(all_projects), quiet=opt.quiet
+                f"Checkout {nb}", len(all_projects), quiet=opt.quiet
             ),
         )