Availability of rez package in build environment using Python version different to the rez installation

[Anti-Distinctlyminty]

I installed rez using python-3.9. I am now trying to use a rez build context that uses python-3.7 and also wishes to use some functions from the rez python library, but I notice that my build fails with:

    from rez.exceptions import BuildError
ModuleNotFoundError: No module named 'rez'

The relevant snippet of package.py

build_requires = ["python-3", "sesi_download_api"]
requires = ["~python-3.7"]

It seems that the rez library is always available in the build context when using the same version of python that was used to install rez? Is the correct way to always request rez for the build

build_requires = ["python-3", "sesi_download_api", "rez"]
requires = ["~python-3.7"]

?