[Numpy-discussion] Remove sctypeNA and typeNA from numpy core

[Matti Picus]

numpy.core has many ways to catalogue dtype names: sctypeDict, typeDict 
(which is precisely sctypeDict), typecodes, and typename. We also 
generate sctypeNA and typeNA but, as issue 11241 shows, it is sometimes 
wrong. They are also not documented and never used inside numpy. Instead 
of fixing it, I propose to remove sctypeNA and typeNA.

Any thoughts or objections?
Matti