[Bug]: CheckBoxes should be square, not rectangular #24471

Bug summary

In #24428 I noted that RadioButtons used to be elliptical, and I fixed that in #24455 to make them circular while only emitting a deprecation warning if users tried to access the .circles attribute (and even then, they directly go back to the old behavior).

CheckButtons have the same issue: they are rectangular if the parent axes is not square. This issue can (most likely) be fixed in exactly the same manner as #24455 (see #24428 (comment)), preferably again keeping the backcompat layer and making things transparent to end users who don't try to access individual artists.

Tagging as good first issue it should(?) mostly be doing the same thing as #24455, but medium difficulty as you need to wrangle a bit with artists and transforms.

Code for reproduction

https://matplotlib.org/stable/gallery/widgets/check_buttons.html and make the window very wide

Actual outcome

Checkboxes become rectangular.

Expected outcome

They stay square.

Additional information

No response

Operating system

any

Matplotlib Version

3.7.0.dev691+g218cdffb53

Matplotlib Backend

any

Python version

3.10

Jupyter version

ENOSUCHLIB

Installation

git checkout