software/slapos-sr-testing: fix python-interpreter for python3

The initial intent was to use an interpreter named `python_for_test` and not
`python`, so that software do not accidentally pick up this python during
compilation.
6033e4fa (software/slapos-sr-testing: fix python3 profile, 2020-12-01)
was a quick fix for some python3 compatibility issues discovered after merge,
but it was not correct, because by using ${python-interpreter:eggs}, it was
installing the default python-interpreter section, which has an interpreter
named `python`.

This caused issues while building proxysql, which assumes /usr/bin/env python
is python2.

The fix is to override python-interpreter directly, we don't need another
section eggs here.

software/slapos-sr-testing/software-py3.cfg

@@ -5,7 +5,7 @@ extends =
 [python]
 part = python3
 
-[eggs]
+[python-interpreter]
 eggs +=
 # plantuml 0.3.0 is only available for Python 3
   ${slapos.test.plantuml-setup:egg}

software/slapos-sr-testing/software.cfg

@@ -167,10 +167,8 @@ branch = master
 egg = slapos.core
 setup = ${slapos.core-repository:location}
 
-[eggs]
-<= python-interpreter
+[python-interpreter]
 eggs =
-  ${python-interpreter:eggs}
   ${lxml-python:egg}
   ${slapos.core-setup:egg}
   ${pillow-python:egg}
@@ -217,7 +215,7 @@ PyPDF2-patch-options = -p1
 
 [eggs/scripts]
 recipe = zc.recipe.egg
-eggs = ${eggs:eggs}
+eggs = ${python-interpreter:eggs}
 scripts =
   slapos
   supervisord