Migrate metadata to setup.cfg

The purpose of this MR is to migrate the metadata in setup.py, requirements.txt, optional_requirements and testing_requirements.txt to setup.cfg and pyproject.toml. In doing so, we also switched from using versioneer to control versioning to setuptools-scm. We have also changed the recommended way of installing a development branch of PESummary (python setup.py install) to pip install . since setup.py install is deprecated.