Use python3 instead of python in the build/test scripts

The scripts do not support python2.

Co-authored-by: David SAUVAGE - AdaLabs Ltd <[email protected]>

Author: Jicquel

core/gnatcoll_core.gpr.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 from __future__ import annotations
 from typing import TYPE_CHECKING
 import logging

gprproject/__init__.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 from .gprbuild import GPRTool
 from subprocess import CalledProcessError
 import argparse

minimal/gnatcoll_minimal.gpr.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 from __future__ import annotations
 from typing import TYPE_CHECKING
 import sys

projects/gnatcoll_projects.gpr.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 from __future__ import annotations
 import logging
 import sys