graphics/py-pycairo: rename and update to 1.29.0

Changelog: https://pycairo.readthedocs.io/en/latest/changelog.html#v1-29-0

Notably, experimental support for free-threaded Python (3.13t, soon 3.14t)

Port and PKGNAME renamed to match Python package metadata, and to
properly build with USE_PYTHON=pep517. Test suite also now exposed.
Remove PORTSCOUT since this does not follow the even-odd version
split.