Add poetry as an env manager

[pjbull]

Adds support for poetry as an env/dependency manager.

• Add poetry to dev requirements for testing
• Add poetry option to dep manager
• Use pyproject.toml project.dependencies section as dependency format (per poetry docs)
• Add poetry test harness
• Add poetry to CI
• Update documentation

Also bumps version in preparation for release

Closes #375

[jayqi]

Looks good.

I didn't realize Poetry finally added support for PEP 621 [project] metadata late last year, finally.

One thing to note is that we might want to swap in poetry-core as the build backend, instead of flit-core. I don't think this is strictly necessary since we don't do anything Poetry-specific in our generated pyproject.toml. I've confirmed that it works as is with flit-core, since the package we generate follows the common part of the specification.

[pjbull]

One thing to note is that we might want to swap in poetry-core as the build backend, i

Makes sense. I updated that. If tests pass, I will merge and release.