A couple of things drive this change—first, PEP621 has been around for a
while now and Poetry shows no signs of adopting it, so while I don't
want to ditch Poetry support (it's very popular), I do want a way to
generate that style of metadata. PDM seems like a good target for that.

Second, Poetry has deprecated some plugins we use... or something. I
didn't look into it, but we're getting warnings on every project build
now. So, seems like time to switch over to something else. Conveniently,
this will make the development commands shorter (`poetry run task check`
becomes `pdm check`).

TODO:

- [x] Convert this project to PDM
- [x] Support PDM metadata (which is partially [PEP
621](https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#writing-pyproject-toml)

)
- [x] Update release workflows to use PDM as well
- [x] Document using PDM metadata in README
- [x] Double-check that `py.typed` is included with a generated PDM
project
- [x] Add snapshot tests for different metadata types and test them as
much as possible

---------

Co-authored-by: Dylan Anthony <dbanty@users.noreply.github.com>