For some reason using cd to blender's python bin folder and then executed command was not working, but ... path to blender's bin folder with python3.7m + command executed at once worked just fine.
Do I have a "pip"?
I don't know if 2.80 was distributed without a pip (as mentioned above), but 2.79 and also 2.83 Mac build containes a pip ... ...
For me the addon installation worked with:
and blender 2.90 and blender 2.83.4.
But under File->Import there is no *.mhx2 option.
It will openable with shortcut n in the toolshelf.