Dependencies with constraints file

This is pretty much an edge case but it seems that some projects need to be built with a pip constraints file, such as Airflow (detailed description as to why here).

The equivalent on the command line with pip would be to run e.g.:

pip install "apache-airflow[celery]==2.2.2" --constraint "https://raw.githubusercontent.com/apache/airflow/constraints-2.2.2/constraints-3.6.txt"

I'm unsure which syntax / design would make most sense here. In our use-case, the dependency is required at build time as well in order to use the library in tests.