Use custom python to install the eggs.

software/backupserver/software.cfg

 [buildout]
 extends =
+  ../../component/lxml-python/buildout.cfg
+  ../../component/python-2.7/buildout.cfg
+  ../../component/python-cffi/buildout.cfg
+  ../../component/python-cryptography/buildout.cfg
   ../../component/dash/buildout.cfg
   ../../component/dcron/buildout.cfg
   ../../component/logrotate/buildout.cfg
@@ -37,9 +41,19 @@ parts =
 [extra-eggs]
 recipe = zc.recipe.egg
 interpreter = pythonforrssgen
+python = python2.7
 eggs =
   PyRSS2Gen
 
+[lxml-python]
+python = python2.7
+
+[python-cffi]
+python = python2.7
+
+[python-cryptography]
+python = python2.7
+
 [networkcache]
 # signature certificates of the following uploaders.
 #   Romain Courteaud