Use a more stable approach for the list of versions to install
Today we always run pip-compile
to regenerate the list of versions, but that lead to instabilities and hacks after our dependencies stopped supporting Python 2 (see #1 (closed), !42 (merged) etc).
The plan is to switch to Python 3 ASAP, but not too early, so, as suggested in !42 (comment 3312252), we should freeze the dependencies.