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

We had a mechanism to catch usage of system python2, but using not
for system python3, which cause the same kind of problems.

software/slapos-sr-testing/software.cfg

@@ -23,7 +23,7 @@ extends =
 
 parts =
   eggs/scripts
-  python2.7-disabled
+  system-python-disabled
   slapos-cookbook
   template
 
@@ -272,8 +272,8 @@ branch = master
 egg = slapos.core
 setup = ${slapos.core-repository:location}
 
-[python2.7-disabled]
-# An "intentionally broken" python2 command that should catch
+[system-python-disabled]
+# An "intentionally broken" python command that should catch
 # accidental usage of things like #!/usr/bin/env python2
 recipe = zc.recipe.egg
 # we need an egg to generate a script, use the one from this part's recipe
@@ -286,10 +286,11 @@ entry-points =
 scripts =
   python
   python2
+  python3
   python2.7
 initialization =
   import sys
-  print("Error: attempt to use system python2", file=sys.stderr)
+  print("Error: attempt to use system python", file=sys.stderr)
   sys.exit(2)
 
 [recurls-repository]