Restore _AxesStack to track a Figure's Axes order.

[QuLogic]

PR Summary

This is a simplified version of the AxesStack removed in #19153, but removing the key entry, and just tracking the index with the Axes. We no longer need to track keys.

Fixes #19598.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [n/a] New features are documented, with examples if plot related.
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[jklymak]

probably should test this since it seems someone could change this?

[QuLogic]

I'll check further, but this may not ever happen, as the Figure.add_* might handle that already now.

[QuLogic]

There are 3 effects of this early return:

1. A different return value, but _AxesStack.add is only called from _add_axes_internal, and that doesn't care about the return value.
2. The Axes doesn't get added to the stack multiple times, but that's what we want anyway.
3. The Axes would not be made current (i.e., moved to the end), but there's a sca in _add_axes_internal, so we don't need to worry about that.

I thought some tests checked add_axes/add_subplot with an existing Axes, but it appears not if this isn't getting coverage. I will add some more tests and remove the return value, which no-one uses.

[tacaswell]

To summarize?

• we used the stack to track three things: what Axes are in the Figure, the order they were added in, and which one is the "current" figure. When we removed the key-tracking we lost the ability to track the order they were added in inadvertently.
• Long term it may be simpler to switch to a list and pointer, but this is the lightest touch fix to get back the old behavior.

[lpsinger]

Why is this method removed?

[QuLogic]

It was moved from _AxesStack to the base class; now that _AxesStack is back, it's not needed there. Better not to add more public API if not necessary.

[jklymak]

I'll wait until today's meeting to merge in case there are any other issues but I think this is good.

[QuLogic]

So we can merge now?

[jklymak]

yes 😄

[QuLogic]

I don't know why meeseeks is randomly not backporting.

@meeseeksdev backport to v3.4.x