Uncertainty parsing considerations

[MichaelTiemannOSC]

Following up on hgrecco/pint#1797:

When interfacing with RESTful APIs, it can happen that we need to translate uncertain quantities to/from strings.

I, too, had the idea of simplifying the task of parsing '+/-' as unicode "±" but decided not to because the Python tokenization library treats unknown characters as ERRORTOKEN, which did not seem like a place to go.