Trust notebook & revert to Python 3.7.9

[CasperWA]

Currently, the Materials Cloud dokku does not support Python 3.7.10.

@ltalirz How do we make sure the buildpacks are updated according to the most updated stack?

[ltalirz]

@ltalirz How do we make sure the buildpacks are updated according to the most updated stack?

The first thing to check would be to use the latest herokuish version by upgrading mc_dokku_herokuish_version accordingly.

We had to pin this version since voila's issue with /dev/random forced us to build our own modified docker images.
(see https://github.com/materialscloud-org/openstack-ansible/blob/master/roles/mc_dokku/templates/random-Dockerfile and https://github.com/materialscloud-org/openstack-ansible/blob/master/roles/mc_dokku/tasks/fix-dockerfile.yml)

[ltalirz]

I've also opened gliderlabs/herokuish#659 which should allow us to get rid of this workaround.