entry: Consistently call python3

This may not be needed in practice, but it is cleaner to align all
shebangs.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/__main__.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # kas - setup tool for bitbake based projects
 #

kas/kas.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # kas - setup tool for bitbake based projects
 #