rmboard: Move to Python 3

This script already works with Python 3. Make it use that by default so
that it can import the patman libraries.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/rmboard.py

@@ -1,4 +1,4 @@
-#! /usr/bin/python
+#! /usr/bin/python3
 # SPDX-License-Identifier: GPL-2.0+
 # Copyright 2019 Google LLC
 #