So the issue was resolved for me by installing sassc (and libsass) via yum and abandoning rubygem-sass (thereby also updating COMPRESS_PRECOMPILERS to use 'sassc'). From what I debugged, the import module error is misleading in that the compressor code was trying to exec the 'sass' command but failing; if I try exec'ing the exact command the compressor attempts i.e., /bin/sass (from rubygem-sass) it fails with:
$ sass -t compressed /opt/mailman/mm-take2/venv-take2/lib/python3.6/site-packages/hyperkitty/static/hyperkitty/sass/hyperkitty.scss /tmp/tmpatncz9d9.css Error: Invalid CSS after "...lor}: #{$value}": expected "{", was ";" on line 4 of /opt/mailman/mm-take2/venv-take2/lib/python3.6/site-packages/hyperkitty/static/hyperkitty/libs/bootstrap/stylesheets/_root.scss from line 11 of /opt/mailman/mm-take2/venv-take2/lib/python3.6/site-packages/hyperkitty/static/hyperkitty/libs/bootstrap/stylesheets/bootstrap.scss from line 4 of /opt/mailman/mm-take2/venv-take2/lib/python3.6/site-packages/hyperkitty/static/hyperkitty/sass/hyperkitty.scss Use --trace for backtrace. $ echo $? 65
However, using sassc it works (from the command line and when re-running django-admin compress):
$ sassc -t compressed /opt/mailman/mm-take2/venv-take2/lib/python3.6/site-packages/hyperkitty/static/hyperkitty/sass/hyperkitty.scss /tmp/tmpatncz9d9.css $ echo $? 0