devel/py-ordered-set: convert to USE_PYTHON=pep517

setup.py only still exists for some kind of legacy compatibility;
the package itself uses flit-core to build. The correct pytest
configuration is included in the PEP-517 configuration.

No functional changes.

While here, update WWW

PR: 280661
Approved by: antoine (maintainer, via PR assignment)
Differential Revision: https://reviews.freebsd.org/D46235