repo: use str(Path) for Python 3.5 a little longer

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

Change-Id: Ieb471b917c5ef5b9cb20b321305c4c13af675109
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2575635
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>

repo

@@ -48,7 +48,7 @@ def main(argv):
   if argv and argv[0] == 'sync':
     _UpdateDepotTools()
 
-  os.execv(REPO, [sys.argv[0]] + argv)
+  os.execv(str(REPO), [sys.argv[0]] + argv)
 
 
 if __name__ == '__main__':