build: switch to python3

Refs: #5095
Change-Id: I7e7a3a5cd49d69c6e389060ccf8fe30599ebd876
diff --git a/waf b/waf
index 7ceee16..a1c5c96 100755
--- a/waf
+++ b/waf
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # encoding: latin-1
 # Thomas Nagy, 2005-2018
 #