Switch transitive dependencies to constraints.txt

[ralgozino]

pip has a feature called constraints that can be used as a lock file to specify version of dependencies that may or may not be installed. See:

https://pip.pypa.io/en/stable/user_guide/#constraints-files
https://luminousmen.com/post/pip-constraints-files

We should use the --constraints feature instead of installing everything as a requirement. Luckily we already have separated files for first-order and transitive dependencies.