MAINT: Remove ._is_array since there's now: isinstance(x, sparray)

[ivirshup]

Reference issue

Closes #18921

What does this implement/fix?

Replace x._is_array with isinstance(x, sparray)

Additional information

May require changes in other open PRs.

I don't know if this needs deprecation since it's private, but it would be super easy to have. E.g.:

@property
def _is_array(self) -> bool:
    warnings.warn(FutureWarning("The `._is_array` attribute is deprecated. Please use `isinstance(x, scipy.sparse.sparray)` instead"))
    return isinstance(self, sparray)

[tupui]

Thank you Isaac. I did a quick search on GitHub and it seems like _is_array is not used that much out there. So +1 for me to not go through a deprecation period.

[rgommers]

Thanks @ivirshup!

I don't know if this needs deprecation since it's private,

So +1 for me to not go through a deprecation period.

Agreed. this seems perfectly safe to remove straight away. We don't need deprecations for private API, and given that this starts with an underscore, that's about as private as it gets.

[tupui]

Alright then let's get this in. CI is happy and the changes LGTM, tests have been updated correctly too.