The NPY_CHAR is not a "real type".   There are no type-coercion functions attached to it nor ufuncs nor a full dtype object.      However, it is used to mimic old Numeric character arrays (especially for copying a string).  

It should have been deprecated before changing the ABI.  I don't think it was realized that it was part of the ABI (mostly for older codes that depended on Numeric).   I think it was just another oversight that inserting type-codes changes this part of the ABI.     

The positive side is that It's a small part of the ABI and not many codes should depend on it.   At this point, I'm not sure what can be done, except to document that NPY_CHAR has been deprecated in 1.7.0 and remove it in 1.8.0 to avoid future ABI difficulties.    

The short answer, is that codes that use NPY_CHAR must be recompiled to be compatible with 1.6.0.