numpy_support: always support [u]int64

Using the variable key also overwrites other keys where long is 32 bits.
NumPy has 64 bit support all the time, so always map it to something.

Change-Id: I44fbbb64b3afe275343016ebe1583309b87fa11f
numpy_support: raise an error if a type cannot be found

Better than None popping up later in the code.

Change-Id: I60b8e9b711b309ac14297d462148c1008e19024d
Merge topic 'numpy-int64-support' into master

00877bc2 numpy_support: raise an error if a type cannot be found
28c46333 numpy_support: always support [u]int64