repo: avoid fstrings for Python 3.5 a little longer

Give people one more month before we make 3.6 a hard requirement.

Change-Id: I696bf3f0edbe64d6294dd626fc9ff16c9e44fd87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2570017
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>

repo

@@ -38,8 +38,8 @@ def _UpdateDepotTools():
   if UPDATE_DEPOT_TOOLS.exists():
     subprocess.run([UPDATE_DEPOT_TOOLS], check=True)
   else:
-    print(f'warning: {UPDATE_DEPOT_TOOLS} does not exist; export '
-          'DEPOT_TOOLS_UPDATE=0 to disable.', file=sys.stderr)
+    print('warning: %s does not exist; export DEPOT_TOOLS_UPDATE=0 to disable.'
+          % (UPDATE_DEPOT_TOOLS,), file=sys.stderr)
 
 
 def main(argv):