gh-80480: Emit DeprecationWarning for array's 'u' type code

[hugovk]

The "u" type code for array was deprecated in Python 3.3 but only via documentation: https://docs.python.org/3.10/library/array.html

Let's add the DeprecationWarning.

It was originally planned to be removed in Python 4.0, but that was before we knew if 3.10 or 4.0 would come after 3.9:

'4.0' is a stand-in for 'sometime after 2.7.final', scheduled for Jan 2020.

#80480 (comment)

So as this is going into 3.12 3.13, let's target 3.14 3.15 3.16 for removal.

• Issue: array: Add 'w' type and deprecate 'u' type. #80480

[asedeno]

As a user of the 'u' array type, I'd like to see this postponed until a UCS4 'w' array type is in place to migrate to, as was proposed in #80480 (comment).

[smontanaro]

(I'm working my way through some PRs which have been approved and are labeled "awaiting merge", hence my seemingly bolt from the blue comment. Why? Read here.)

Looks like this was approved before @asedeno's comment. Can someone respond to the expressed concern? (It seems no 'w' code has been implemented yet.)

[hugovk]

Thanks for the ping. I commented in the issue, let's continue there.

[hugovk]

Updated to target removal in 3.16, following addition of 'w' in #105242 and following the plan at #80480 (comment).

[vstinner]

LGTM. I just left minor remarks on the doc. It's up to you to address them or ignore them ;-) Thanks for the updates.