Deprecation raised during the install of CRDS

[WilliamJamieson]

When one installs CRDS on python 3.11 using pip 23.0.1 the following warning gets issued:

DEPRECATION: lxml is being installed using the legacy 'setup.py install' method, 
because it does not have a 'pyproject.toml' and the 'wheel' package is not installed. 
pip 23.1 will enforce this behaviour change. A possible replacement is to enable the 
'--use-pep517' option. Discussion can be found at https://github.com/pypa/pip/issues/8559

This appears to be because lxml relies only on a setup.py to control its install, so there is no wheel generated.

This may start creating issues for installing CRDS (and its downstream packages) on pip 23.1+, which seems like it will necessitate adding the --use-pep517 flag to pip install in these cases. This may create issues for our users in the near future.

There seems to be a bug reported to lxml: https://bugs.launchpad.net/lxml/+bug/1993882, but it does not appear to have much activity in attempting to fix it.