Support for pip --constraint

Is your feature request related to a problem? Please describe.

Some projects recommend installing using pip with --constraint: pip install
apache-airflow==1.10.12
–constraint “https://raw.githubusercontent.com/apache/airflow/constraints-1.10.12/constraints-3.7.txt”

It would be nice to have an equivalent pipenv command. The idea is that some project already provide all the resolved dependencies (an equivalent of the Pipfile.lock file) that should be input to the dependency resolution step.

Describe the solution you’d like

Add a --constraint parameter to pipenv that behaves as pip --constraint does.

Describe alternatives you’ve considered

I think there is no alternative but to drop the --constraint and hope that the dependencies’ version resolved by pipenv match the ones that in the project’s constraints.

Additional context

[seawolf42]

@uranusjr a case in which a constraint should not be placed in the Pipfile is when the constraint is intended to be relative to the lock file. In a monorepo situation, it is desireable to have a single Pipfile control dependencies as a whole and then have subsystems each list their own minimal list of requirements… pip --constraints <...> allows you to indicate that for that subsystem, if a dependency is explicitly or implicitly listed as a dependency, then the version used shall be the version in the constraints file, but if it isn’t listed as a dependency then it should not be. It would be very helpful if pipenv supported this as well.

[uranusjr]

I feel it would be much more explicit (and reproducible) if the constraints are put directly into Pipfile. Maybe what we should do instead is to have a way to sync constraint files and Pipfile, instead