Pynucos

The unit tests now require Python 3.7 and above.

The rest of the code will probably still work on Python2 -- but that won't be tested or maintained anymore.

Other changes:

- Conversion value for Specific Gravity at 15C has been changed in the fourth digit to match the standard used by the Petroleum Industry.

- Tests have been cleaned up to use pure pytest - no more unittest tests.

Fixed pathlib and Unicode issues on Python2

A couple new units, and setting the unicode versions as the "canonical" abbreviation.

Also added an RST page for all the units.

Put the Unicode per mille sign in where appropriate as the canonical abbreviation.

updated unit data with the micron symbol where appropirate.

re-ordered synonyms so that the first one is the "standard" short form.

Added Pressure units

(and removed adhesion)