poetry to setuptools; min py38

[loriab]

Description

• After all-but-py37 builds spontaneously broke in the last few days, it's easier to move the build backend back in line with other QCA projects (QCFractal and QCManyBody) rather than find the issue with poetry's solve-all-at-once and highly-version-specified format.
• This should hit most poetry uses and examples. I've already confirmed that new builds on master didn't break, so fine targeting this to next2024.
• Dialed back the pydantic versions for docs build. There's some errors with latest pydantic, so that pin needs investigating and releasing.
• Also bump the min python to 3.8+. Even that is end-of-life but QCEngine test lanes still use it.
• There will likely be refinements and corrections to this switchover, but this is a largely working start.

Changelog description

Status

• Code base linted
• Ready to go

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.17%. Comparing base (a1f7dcc) to head (72c4fce).
Report is 5 commits behind head on next2024.

Additional details and impacted files

[loriab]

Requesting review @davidbrownell, @krachwal, @rfievet, @ketanbj, @varun646, @jyoung3131