Drop glue.ligolw based on Python library version, then remove completely in 4.0.0

I would like to propose dropping glue.ligolw based on the Python library version, so that Python 3.10 and beyond doesn't include it (which would resolve the issues shown in !87 (merged)), but users who really need it can still get a supported library on Python <=3.9.

At the same time we should mark that glue.ligolw will be removed completely with lscsoft-glue 4.0.0 (presuming the next release is 3.0.0), so that users know to pin to <4.0.0 if they really need glue.ligolw. Note that gwpy, ligo.skymap, and python-ligo-lw itself all provide ways to translate old XML documents into something that can be read using the new python-ligo-lw library, so glue.ligolw is not required to simply read an old document (it would be required to write an old-style document).

This is my way of dropping support for glue.ligolw in the nicest possible manner.

I am happy to implement the necessary build changes to support this, as soon as !89 (merged) is merged.