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

Author: mikofski

pvlib/singlediode.py

@@ -13,7 +13,8 @@
 try:
     from scipy.optimize import brentq
 except ImportError:
-    brentq = NotImplemented
+    def brentq(): raise NotImplementedError(
+        "brentq can't be imported because scipy isn't installed")
 
 # FIXME: change this to newton when scipy-1.2 is released
 try: