Use Python 3 instead of Python 2.

Upstream-Status: Pending
Signed-off-by: Ross Burton <ross.burton@intel.com>

diff --git a/waf b/waf
index 6ce2a24..de3c898 100755
--- a/waf
+++ b/waf
@@ -1 +1 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3