Use poetry-core for build

[harens]

This should hopefully close #568 by switching from poetry to poetry-core. See python-poetry/poetry#1975 for more details on the issue.

This is done as recommended by the Poetry docs:

If your pyproject.toml file still references poetry directly as a build backend, you should update it to reference poetry_core instead.

I couldn't test this on my machine as a result of pyca/cryptography#3489 occuring with poetry. Hence I marked it as a WIP.

The only thing I'm a bit cautious of is the use of setuptools in the build-system. I don't think it's required for poetry (see python-poetry/poetry#2956 (comment)) but I'm not sure about poetry-core. However, there are other projects with a similar setup.py without needing setuptools in the pyproject.toml, such as rich, so I thought I'd give it a shot and revert it if necessary.

See https://trac.macports.org/ticket/62682 for full discussion.

[harens]

It seems like tests are failing for unrelated reasons relating specifically to HTTPERROR (maybe #490?).

[commonism]

Glad to see this fails for you as well.
I'm having the similar issues (#786), but I actually touched relevant authorization code, so … seems like something else broke.

[adferrand]

Just to keep you informed that I follow the discussions, I will invest myself as soon as I have some time.

[adferrand]

I update the PR with master, and tests are passing. LGTM!

[harens]

Thanks @adferrand.