MAINT: CPython (and maybe other dep) runtime version checks

[tylerjereddy]

It probably makes sense to provide a clear error message when using too old (unsupported) dependencies at runtime, that we are not testing in CI, for example old versions of NumPy, sklearn, etc.. We can find examples of how to do this properly with PEP440 in upstream projects like SciPy, sklearn, etc.

For suggested approach/timeline for dropping older dependencies, see community document at: https://scientific-python.org/specs/spec-0000/ .