Use python3 binary rather than unversioned python

Some distros do not provide an unversioned python binary and it could
point to either python2 or python3. Use python3 explicitly instead.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>

tools/keymap-gen

diff --git a/tools/keymap-gen b/tools/keymap-gen
index 32407ac..b6cc95b 100755
--- a/tools/keymap-gen
+++ b/tools/keymap-gen
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 # -*- python -*-
 #
 # Keycode Map Generator