Consider using poetry instead of flit

[blakeNaccarato]

There may be fewer growing pains if we bite the bullet and use poetry. Although I am concerned about user-friendliness to newcomers. Perhaps instruction about installing poetry with pipx would help somewhat. After all, copier must already be installed with pipx.

• Migrate to poetry
• Include standard PyPI metadata like Python version, docs page, issues page, etc.

[blakeNaccarato]

Another reason for this is hashing of dependencies to avoid supply chain attacks where a dependency is yanked and replaced with a malicious version.