Using a unit that is not listed in units.py raises a misleading Exception message

[tluth]

After upgrading to the latest version and trying to pull down a commit object, I noticed I was getting the exception:
"Invalid units: expected type str but received <class 'str'> (squaremperperson)." which is obviously a bit confusing in itself, and I had to look through the code to realise that it's not actually related to types but just the fact that I used a unit which isn't recognised by speckle.

specklepy/src/specklepy/objects/units.py

Line 59 in 420c73f

raise SpeckleInvalidUnitException(unit)