software/slapos-sr-testing: correctly disallow usage of system python3

previous patch - 24f5a96c (software/slapos-sr-testing: disallow usage
of system python3, 2023-10-02) - was incorrect, an `entry-point` is
required for a script to be generated.

software/slapos-sr-testing/software.cfg

@@ -274,7 +274,7 @@ setup = ${slapos.core-repository:location}
 
 [system-python-disabled]
 # An "intentionally broken" python command that should catch
-# accidental usage of things like #!/usr/bin/env python2
+# accidental usage of things like #!/usr/bin/env python
 recipe = zc.recipe.egg
 # we need an egg to generate a script, use the one from this part's recipe
 eggs = ${:recipe}
@@ -283,11 +283,12 @@ entry-points =
   python=${:eggs}:ignored
   python2=${:eggs}:ignored
   python2.7=${:eggs}:ignored
+  python3=${:eggs}:ignored
 scripts =
   python
   python2
-  python3
   python2.7
+  python3
 initialization =
   import sys
   print("Error: attempt to use system python", file=sys.stderr)