textproc/py-python-xmp-toolkit: Update to 2.1.0

• Switch to the PEP517 build framework.

• Add TEST_ENV to ensure that tests run against the staged copy and switch to USE_PYTHON=pytest.

Changelog since 2.0.1:

https://github.com/python-xmp-toolkit/python-xmp-toolkit/blob/v2.1.0/CHANGELOG