stack/lamp: fix using python3

We were using python2 after extending python2, this explicitly set
python:part = python3

stack/lamp/buildout.cfg

@@ -31,6 +31,9 @@ extends =
   ../../component/apache-php/buildout.cfg
 
 
+[python]
+part = python3
+
 [template-download-base]
 recipe = slapos.recipe.build:download
 url = ${:_profile_base_location_}/${:filename}