babel.units.format_unit does not fall back from long to short correctly

[trillllian]

>>> import babel.units
>>> babel.units.format_unit(1, 'duration-month', 'long', locale='et')
'1 duration-month'  # should be: '1 kuu'

In CLDR for estonian, duration-month is only defined for 'short' and 'narrow' lengths. Before CLDR 44, it was also defined for 'long', but since then it has been aliased to the short version. It seems that pybabel does not understand these aliases properly, cf #1075, but the fix there was only for babel.dates, not for babel.units.

This happens for other locales and other (non-duration) units too.