support using requires-python to set python-version for minimum dependencies

[minrk]

Problem

minimum dependencies defaults to using Python 3.8. It would be nice if it defaulted to reading the minimum Python version in pyproject.toml.

Proposed Solution

parse pyproject.toml looking for project.requires-python (or wheel metadata looking for the equivalent field) instead of hardcoding Python 3.8

Additional context

came up bumping minimum Python to 3.9 in ipykernel, where the minimum dependency test started failing due to using an unsupported Python 3.8.