ENH: Allow buffer export and pickling of `np.generic` subclasses

Previously we had code that would allow exporting the buffer, but
then fail for any reasonable subclass, because such a subclass
should have its own user-dtype.
The change is, that now a subclass without its own user-dtype will
inherit the correct behaviour directly.
This allows pickling of of such user-defined scalars (with
user-defined dtype) if no FORMAT was requested in the buffer export.

The latter allows the generic pickling code to succeed.

Closes gh-17294

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