Convert to the more modern and preferred pyproject.toml configuration
for the project.
- Add the majority of the configuration to pyproject.toml.
- Remove setup.cfg as it is no longer needed.
- Update tox.ini to remove a lingering testenv for Python 2.7 which is
no longer used.
NOTE: Manifest.in is left as the files it includes are outside of the
package directories.
- Drop support for Python 2
- Drop support for Python 3.7, 3.8 is new minimum
- Update tox to include newer python version and drop old ones
- Update GitHub action for python version changes, use standard python
setup action
- Update GitHub action to use pinned action versions
- Update version to 2.0.0-dev
