software/buildout-testing (py2): Downgrade pip, wheel and setuptools

This SR is kept separate because it is not part of SR tests.

software/buildout-testing/software-py2.cfg

@@ -2,5 +2,5 @@
 extends =
   software.cfg
 
-[python]
-part = python2.7
+# Python2 versions for buildout (keep last)
+  ../../stack/slapos-py2.cfg