BUG: fix brentq doesn't raise error in singlediode (#556)
BUG: fix brentq doesn't raise error in singlediode (#556)

* if `brentq` is called in `singlediode.py` but no SciPy, then should raise an exception
* currently using `NotImplemented` raises TypeError: 'NotImplementedType' object is not callable because `NotImplemented` is a constant
mikofski/pvlib-python,wholmgren/pvlib-python,alorenzo175/pvlib-python,pvlib/pvlib-python,rubennj/pvlib-python,anomam/pvlib-python,cwhanse/pvlib-python