Dependency on lxml breaks CI on Python 3.11

[jaraco]

Once again, I'm finding the lxml dependency means the CI pipelines are broken in Python 3.11 until lxml provides binary builds for that Python version.

Originally posted by @jaraco in #24 (comment)

[jaraco]

Last year, it took ~3 months to get wheels (https://bugs.launchpad.net/lxml/+bug/1929359).

[jaraco]

Filed https://bugs.launchpad.net/lxml/+bug/1977998 to track the need for Python 3.11.

[jaraco]

This issue is annoying, because lxml is not a runtime dependency of cssutils, but is used by a couple of tools/examples files. One thing we could do:

• Make lxml conditionally installed only on Python < 3.11 and when lxml isn't present, skip the testing of those modules that import lxml.

[jaraco]

Workaround is sufficient for now.

[jayaddison]

A note that I think this might be re-resolvable (for most platforms) by unpinning the lxml dependency in setup.cfg.

One catch to look out for, though: some versions of MacOS appear to have trouble building lxml from source (which they attempt by default during installation of that package) -- but are generally fine when using prebuilt binary wheels.