> Constraints files are requirements files that only control which version of a requirement is installed, not whether it is installed or not. Their syntax and contents is nearly identical to Requirements Files. There is one key difference: Including a package in a constraints file does not trigger installation of the package.
ERROR: You must give at least one requirement to install (see "pip help install")
So it seems like a strange choice of usage example. You have to provide both requirements and constraints for it to do anything useful (applying the version constraints to the requirements and their dependencies).
> Constraints files are requirements files that only control which version of a requirement is installed, not whether it is installed or not. Their syntax and contents is nearly identical to Requirements Files. There is one key difference: Including a package in a constraints file does not trigger installation of the package.
> Use a constraints file like so: