Use python3 as the interpreter instead of python (ie v2)

This fixes lintian warning "unusual-interpreter".

Part of the Debian edition of FreeCAD since 2023.

src/Mod/AddonManager/AddonManager.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # SPDX-License-Identifier: LGPL-2.1-or-later
 # ***************************************************************************