Use new license format in pyproject.toml

[delucchi-cmu]

Change Description

Closes #532

Updates generation of pyproject.toml to use the new format from PEP-639.

In the unit test file, I was about to add a new method, pyproject_toml_is_valid, which would require also calling the initialize_git_repo method in each test method, and I'd had enough. Created a single convenience method for checks that EVERY configuration needs to run.

Checklist

• This PR is meant for the lincc-frameworks/python-project-template repo and not a downstream one instead.
• This change is linked to an open issue
• This change includes integration testing, or is small enough to be covered by existing tests

[gitosaurus]

LGTM