Set up conditional dependency on pandas for py 3.7

[heather999]

I have a simple PR open on qp: LSSTDESC/qp#73 that just removes astropy from the list of dependencies - since it is truly no longer a dependency. Unfortunately the tests against py3.7 fail as the qp setup installs tables_io which installs pandas.
The recent pandas releases since 1.4.x have dropped support for python 3.7.
While I wondered aloud about dropping support for 3.7 - given that python 3.7 is still alive for now and clearly still supported on conda-forge, I'd like to suggest modifying the requirements and setup.py in this repo to conditionally restrict the version of pandas if we're dealing with python 3.7, otherwise, there should be no restrictions.
I think I can muster a PR to make that happen.