scripts/serato_heartbeat: Fix small formatting issue

When merging master into a feature branch, this can causes problems with
black.

scripts/serato_heartbeat.py

@@ -98,7 +98,7 @@ def main(argv=None):
         epilog=__doc__,
         formatter_class=argparse.RawDescriptionHelpFormatter,
     )
-    parser.add_argument("port", metavar='PORT', help="MIDI output port name")
+    parser.add_argument("port", metavar="PORT", help="MIDI output port name")
     parser.add_argument("-f", "--xmlfile", help="Mixxx XML file")
     args = parser.parse_args(argv)