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/list.py

@@ -131,7 +131,7 @@ This is similar to running: repo forall -c 'echo "$REPO_PATH : $REPO_PROJECT"'.
             elif opt.path_only and not opt.name_only:
                 lines.append("%s" % (_getpath(project)))
             else:
-                lines.append("%s : %s" % (_getpath(project), project.name))
+                lines.append(f"{_getpath(project)} : {project.name}")
 
         if lines:
             lines.sort()