Migrate from setup.py/setup.cfg to pyproject.toml and drop Python 3.7 support

[seisman]

Description of proposed changes

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.