Adding API change notes

API change notes for future releases are collected in next_api_changes. They are divided into four subdirectories:

• Deprecations: Announcements of future changes. Typically, these will raise a deprecation warning and users of this API should change their code to stay compatible with future releases of Matplotlib. If possible, state what should be used instead.

• Removals: Parts of the API that got removed. If possible, state what should be used instead.

• Behaviour changes: API that stays valid but will yield a different result.

• Development changes: Changes to the build process, dependencies, etc.

Please place new entries in these directories with a new file named 99999-ABC.rst, where 99999 would be the PR number, and ABC the author's initials. Typically, each change will get its own file, but you may also amend existing files when suitable. The overall goal is a comprehensible documentation of the changes.

A typical entry could look like this:

Locators
~~~~~~~~
The unused `Locator.autoscale()` method is deprecated (pass the axis
limits to `Locator.view_limits()` instead).

Please avoid using references in section titles, as it causes links to be confusing in the table of contents. Instead, ensure that a reference is included in the descriptive text.