build: Use python3 for the post install script

While *some* systems alias python to python3 nowdays, this is
not true for eveything. Especially systems that can potentially
offer both python2 and python3.

According to both PEP 394 and PEP 441 its recommended to always
add the 3 in the shebang.

build-aux/meson/post-install.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import sys