DEP: Raise warnings for deprecated functions PyArray_As1D, PyArray_As2D

[kritisingh1]

Related to #11521

[mattip]

Unfortunately we really cannot remove these from numpy/core/code_generators/numpy_api.py until we release a Numpy2 (which may never happen) as it breaks backward compatibility. We can only raise an exception in the C code if the functions are called.

[kritisingh1]

My bad, @mattip how do we deal with raising exceptions in C side of functions, any previous PR or example?

[mattip]

The return value will be different from each function, and in the general case there may be cleanup to be done (not in this simple case). Since there is no DeprecationError I propose raising a NotImplementedError like this:

PyErr_SetString(PyExc_NotImplementedError, msg);
return <errorval>;

[kritisingh1]

I'm not sure how to go ahead with this since the travis builds are failing due to reasons jargon to me. Based on a discussion with @seberg recently, builds are failing due to presence of unused lines of code in the functions being deprecated. A possible solution is to just set the error message and return -1, and omit all other lines in the body of function. Should I try this?

[seberg]

OK, lets put this in now that the tests pass. thanks Kriti.