Switch to new Debian Buster toolforge Grid and Python3.7 everywhere
The previous Tools Grid, running on Strech, was using Python 3.5.
To use it, we forced the use of a Python3.5
virtual environment for cron jobs (eac4d2c5).
The previous Tools Grid is now deprecated and
we need to move the workload to Python 3.7 [1].
We were already using Python3.7 for the webservice
(since 9d5e608e07), so there is not that much to do.
- Remove the eac4d2c5 workaround, and let the cron job use the normal venv.
- Connect to a Buster Bastion host in Ansible hosts file
- Add -release buster flag to jsub commands in cron jobs
[1] See https://wikitech.wikimedia.org/wiki/News/Toolforge_Stretch_deprecation
Bug: T257942