Fix half-migrated pyproject.toml to be valid and installable by poetry

[dbarnett]

Looks like someone started updating packaging to use poetry but never finished. This finishes consolidating the configs and makes it so poetry build and pipx install . actually work for me.

Note: hatchling or setuptools may be a better choice instead of poetry considering poetry has been so slow to adopt PEP standards and has so much nonstandard config.

[dbarnett]

Sending a PR to your repo instead of trying to update insanum's per insanum#688.