The relevant part of the error was not the one about sassc, it was this other one:
''' CommandError: An error occurred during rendering /usr/local/lib/python3.6/site-packages/django_mailman3/templates/account/email_confirm.html: Error: It's not clear which file to import for '@import "../libs/bootstrap/stylesheets/bootstrap"'. Candidates: ../libs/bootstrap/stylesheets/_bootstrap.scss ../libs/bootstrap/stylesheets/bootstrap.scss Please delete or rename all but one of these files. on line 4 of static/hyperkitty/sass/hyperkitty.scss
@import "../libs/bootstrap/stylesheets/bootstrap"; ''' The _bootstrap.scss was lingering from the previously running version, and bootstrap.scss had been added wth the upgrade. This was confussing the templating engine ...
Mailman 3.3.3 humming happily with Postorius 1.3.4 and Hyperkitty 1.3.4
Thanks for your time!