devel/py-pymaven-patch: switch from devel/py-lxml to devel/py-lxml5

This is the last one which depends on devel/py-lxml. Upstream's
requirements.txt says "lxml>=4.0.0" (no upper bound). So switching to
devel/py-lxml5 should cause no harm.

PR: 288357
Approved by: kai (maintainer)