Get Python version from pyproject.toml

For quite some time we've been able to use [pyproject.toml to specify the Python version](https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#python-requires). and this is widely supported by tooling as well as being part of [PEP 621](https://peps.python.org/pep-0621/), which is a bit better than using an ad-hoc standard such as a `.python_version` file.

It would be nice if we could use this in Heroku.