add pyproject.toml lines to make this a package

[xzackli]

This PR adds some lines to pyproject.toml so that one can install this as a package with

flit install

for i.e. local development.

[buddhasystem]

I defer it to Anze since that's an important step. It sounds great, of course.

[slosar]

I have nothing to complain here!

[slosar]

I don't understand why it doesn't run the github actions workflow. Even when I run it manually it just contines to say "Some checks haven't completed yet." Will override for now, but this needs fixing.

[slosar]

It used to work last time.