REF: de-duplicate hard-coded unit code
I had hoped  to go further and get rid of one of the dtypes.pyx/pxd classes entirely, but ATM i dont think thats feasible.
https://github.com/pandas-dev/pandas/runs/5519720530?check_suite_focus=true mypy complaint fixed; looks like the remaining CI fails are happening on many prs Thanks @jbrockmendel 