repo: restore fstrings for Python 3.6

We're looking at dropping Python 3.5 support from repo.  Lets roll
this back out in Chromium a little early to get a better idea of
who is still relying on it.

Change-Id: Idcdd5b0a957f5a2f8b146149c407c26347be0aa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2830590
Reviewed-by: Stephane Belmon <sbelmon@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('warning: %s does not exist; export DEPOT_TOOLS_UPDATE=0 to disable.'
-          % (UPDATE_DEPOT_TOOLS,), file=sys.stderr)
+    print(f'warning: {UPDATE_DEPOT_TOOLS} does not exist; export '
+          'DEPOT_TOOLS_UPDATE=0 to disable.', file=sys.stderr)
 
 
 def main(argv):