[REGR} formatter does not handle negative powers in numerator correctly

[andrewgsavage]

The doctests were failing. I've changed them to get them working again:

# Old docs
    >>> print(formatter(result[0].items()))
    T * V / L
# Current docs 
    >>> numerator = [item for item in result[0].items() if item[1]>0]
    >>> denominator = [item for item in result[0].items() if item[1]<0]
    >>> print(formatter(numerator, denominator))
    V * T / L

This feels uncessarily clunky

[andrewgsavage]

I've added the first line (commented here) to avoid this error message:
docs\user\formatting.rst

     10 # >>> ureg.formatter._registry = ureg
     11 q = ureg.Quantity(2,"m")
---> 12 str(q)
     13 '2.3e-06 ups!'

File F:\A\repos\pint\pint\facets\plain\quantity.py:270, in PlainQuantity.__str__(self)
    269 def __str__(self) -> str:
--> 270     return self._REGISTRY.formatter.format_quantity(self)

File F:\A\repos\pint\pint\delegates\formatter\plain.py:129, in DefaultFormatter.format_quantity(self, quantity, qspec, sort_func, **babel_kwds)
    122 """Format a quantity (magnitude and unit) into string
    123 given a string formatting specification and locale related arguments.
    124 """
    126 registry = self._registry
    128 mspec, uspec = split_format(
--> 129     qspec, registry.formatter.default_format, registry.separate_format_defaults
    130 )
    132 joint_fstring = "{} {}"
    133 return join_mu(
    134     joint_fstring,
    135     self.format_magnitude(quantity.magnitude, mspec, **babel_kwds),
    136     self.format_unit(quantity.unit_items(), uspec, sort_func, **babel_kwds),
    137 )

AttributeError: 'NoneType' object has no attribute 'formatter'