*: only install backports-lzma on python2

on python3 we use lzma directly

software/slapos-sr-testing/software.cfg

@@ -337,7 +337,6 @@ eggs +=
   ${python-pynacl:egg}
   ${python-cryptography:egg}
   ${python-mysqlclient:egg}
-  ${backports.lzma:egg}
   ${bcrypt:egg}
   ${psycopg2:egg}
   ${selenium:egg}

software/slapos-testing/software.cfg

@@ -141,7 +141,6 @@ eggs +=
   ${lxml-python:egg}
   ${python-PyYAML:egg}
   ${python-cryptography:egg}
-  ${backports.lzma:egg}
   ${pycurl:egg}
   ${bcrypt:egg}
   dnspython

stack/slapos.cfg

@@ -118,7 +118,10 @@ eggs =
   ${lxml-python:egg}
   ${pycurl:egg}
   ${python-cryptography:egg}
-  ${backports.lzma:egg}
+  ${:extra-eggs}
+extra-eggs =
+[slapos-toolbox-dependencies:python2]
+extra-eggs = ${backports.lzma:egg}
 
 # Install a slapos command with networkcache enabled in ${buildout:bin-directory}
 [slapos-command]