Fix python scripts

doc/bash_completion/make_bash_completion.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import subprocess
 from io import StringIO

doc/manual-src/en/mkapiref.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # aria2 - The high speed download utility
 #