Measurement

Major Changes:

* Drop Python <= 3.6 support
* Add Python 3.8 Support
* Add `AbstractUnit` classes to declate units.
* Add all new `AbstractMeasurement` as Measurement base class.
* Refactor `MeasurementBase` to a proper be a metaclass.
* Add support for arithmetic operations on geometrical measures (distance x distance = area, …)
* Add support for arithmetic operations on electromagnetic measures (U x I = R, …)
* Add inductance measure
* Add support for non ASCII units (`m²`, …)

What's Changed
* Fix 75 -- Update packaging and CI to PEP518 by codingjoe in https://github.com/coddingtonbear/python-measurement/pull/771 to

**Full Changelog**: https://github.com/coddingtonbear/python-measurement/compare/4.0a7...4.0a8

- Fixed issue with some factor names containing underscores
- Improved comments and documentation

* Add distance unit point (pt)

* Fix 49 -- Fix "denominator" typos

* Fix 40 -- Raise type error with proper message