@adamchainz @craiga As @tut_tuuut said, there is https://scalingo.com. They use forks of Heroku buildpacks (eg. the Python one https://github.com/Scalingo/python-buildpack) so very familiar.
https://beta.gouv.fr/en/ #Django projects mostly deploy there or on https://www.clever.cloud (not based on Heroku buildpacks). Both have very reactive customer service.