Bug#876921: python-pint FTBFS with python-numpy 1.13.1: test failures

[Christoph Berg]

> TypeError: unsupported operand type(s) for ** or pow(): 'numpy.ndarray' and 
> 'Quantity'

Upstream is working on a fix, tentative ETA is this weekend.

https://github.com/hgrecco/pint/issues/577#issuecomment-366917313

Christoph