slapos-sr-testing: Use backports.lzma

Tests need to open xz files for checking.

software/slapos-sr-testing/software.cfg

@@ -8,6 +8,7 @@ extends =
   ../../component/python-cryptography/buildout.cfg
   ../../component/python-pynacl/buildout.cfg
   ../../component/quic_client-bin/buildout.cfg
+  ../../component/python-backports-lzma/buildout.cfg
 
   ../../stack/slapos.cfg
 
@@ -108,6 +109,7 @@ eggs =
   ${slapos.test.seleniumserver-setup:egg}
   ${slapos.test.slaprunner-setup:egg}
   ${slapos.test.jupyter-setup:egg}
+  ${backports.lzma:egg}
 
 entry-points =
   runTestSuite=erp5.util.testsuite:runTestSuite
@@ -171,3 +173,5 @@ Django = 1.11
 # Required by:
 # selenium==3.141.0
 urllib3 = 1.24.1
+
+backports.lzma = 0.0.13