remove testing support for python3.7

This is the same issue as when python3.6 reached end-of-life: support packages that get installed just for testing get updated and then suddenly break the testing pipeline. For example: this pipeline, which is the same issue as here:

https://github.com/microsoft/LightGBM/issues/5969

Rather than play whackamole or pin every single dependency, I'm just going to remove python3.7 testing support.