Support degree sign

[iamthad]

The degree sign should be supported.

Current behavior:

>>> import pint
>>> units = pint.UnitRegistry()
>>> Q_ = units.Quantity
>>> Q_('90°')
<Quantity(90, 'dimensionless')>

Expected behavior:

>>> import pint
>>> units = pint.UnitRegistry()
>>> Q_ = units.Quantity
>>> Q_('90°')
<Quantity(90, 'degree')>

It does not appear to be possible to just define the degree sign as a unit alias in the unit registry, possibly because the degree sign is not letter-like.

[hgrecco]

Indeed this is due to the fact we use the python tokenizer. Same problem with #429. I would like to see a better way to dealing with all these things.