Updating TOML parsing for Python 3.11

Use tomllib if available, else use toml.

An upcoming release will drop support for toml and therefore become
supported on Python 3.11+ only.

Toolchain updates to accompany migration away from setup.py

Add license file; fix to makefile

Bug fix for multivariate options

Fix Makefile; pipx no longer supports --spec

Documentation; mypy integration into tests

Adding make build

Initial commit