refactor: remove ibis NA

[ncclementi]

• Closes refactor: remove ibis.NA  #9311

This PR removes ibis.NA, replacing it appearances for ibis.null()

Merge after #9300 - I'll rebase based on main once merged

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[ncclementi]

I accidentally change the html from a doc freeze, that had a ibis.NA what's the best way to fix that? I've never generated a frozen page for docs.

[jcrist]

Copying a question from the issue into the PR:

For implementation purposes, do we want to deprecate ibis.NAor remove it as a breaking change. If it's a deprecation, how do we deprecate ibis.NA = null() the @deprecated seems to only work for methods/functions.

I think most users never touch ibis.NA, so I wouldn't be opposed to doing a clean break. In that case though I'd think we'd want to wait until 10.0 since that would definitely be a breaking change. Given fillna/dropna are cleanly deprecated in the other PR, I have a slight preference for adding a warning here as well so they all can be released at the same time.

Two ways I can see to do a cleaner deprecation:

• Expose ibis.NA via a module __getattr__ on the ibis namespace, and warn when that attribute is accessed. This would warn every time ibis.NA or from ibis import NA is executed. The __getattr__ here would just return the result of ibis.null(), the warning is on access not implementation.
• Create a new NAScalar type (like the existing NullScalar) that warns in the .op() method before returning the existing op representation. This would warn every time ibis.NA is used to build up larger expressions.

I think the former is a better deprecation method, but either would work well enough.

[ncclementi]

Since we already had a __getattr__ I went this route to keep the typing for the load_backend portion. Once we deprecate NA completely we can remove the workaround

[ncclementi]

I'll rebase here once we merged #9300

[jcrist]

I think this should work instead, since null is already imported above:

Suggested change

	import ibis
	return ibis.null()
	return null()

[jcrist]

Suggested change

	"Accessing 'ibis.NA' is deprecated as of v9.1 and will be removed in a future version. "
	"Use 'ibis.null()' instead.",
	"The 'ibis.NA' constant is deprecated as of v9.1 and will be removed in a future "
	"version. Use 'ibis.null()' instead.",

[ncclementi]

The amount of conflicts got a bit nasty, it'll be faster to open a new PR and make it cleaner. I'll put this up in a few min.

[ncclementi]

Closing in favor of #9344