Deprecate use of chararray in io.fits

[embray]

As raised in #3854 and earlier issues, Numpy is gradually trying to deprecate and eventually kill off the old Numarray chararray class. This is fine, but io.fits still uses it for FITS tables. This is especially important in supporting FITS ASCII tables, for its ability to automatically rstrip() individual strings returned from the array.

However, for FITS data we can just use our own class that has this capability (we don't need any of the other chararray methods). It might be nice to go ahead and use some version of my encoded_text_array class for FITS string columns (and possible string columns in other Astropy tables), and to add optional rstrip support to that class (it already has automatic rstripping, but that should probably be optional).

I think for backwards compat the new class should inherit chararray methods, but they would all raise deprecation warnings. Similarly isinstance(..., chararray) checks should raise a deprecation warning (that the class will no longer be a chararray subclass after long).

[astrofrog]

Don't think this will happen for 1.1 so removing milestone

[pllim]

Hmm did numpy 2.0 already forced our hands in this issue (maybe #15363 ) or not?

[neutrinoceros]

@pllim I don't think so. In fact, numpy 2.0 has received some attention in that area. For instance:

• ENH: Add replace ufunc for bytes and unicode dtypes numpy/numpy#25171
• BUG: three string ufunc bugs, one leading to segfault numpy/numpy#25515

Besides, np.chararray is still very much used in io.fits, so when the time comes, I'm sure we'll know 😅

[mhvk]

I think the idea is still that we'd slowly stop using np.char and instead directly use functions from the new np.strings. But there is no specific deprecation planned for 2.0 (and the ufuncs are now used inside chararray so we'll benefit from the speed-up).