Deprecate rcParams["datapath"] in favor of mpl.get_data_path().

[anntzer]

The rcParam cannot be meaningfully set by the end user from their
matplotlibrc or Python code.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[anntzer]

Should probably have made it to 3.2 instead due to #16678 (#14401), so I milestoned this to 3.2.1. If we agree about the remilestone I will update the version numbers in the diffs.

[tacaswell]

👍 but the deprecation notes should go in the 3.2 file.

I don't think this is going to backport cleanly as the "pull defaults from the sample file" logic is not on the 3.2.x branch. It may have to be re-written on that branch.

[anntzer]

that's fine, I'll open a separate pr for 3.2.1.
edit: done at #16722; also moved the deprecation note.

[QuLogic]

This needs to be synced with changes in #16722 before merging, or just left for the branch merge up.

[anntzer]

let's wait for that other pr to be merged

[anntzer]

looks like all's left after the rebase is just the changelog note?

[tacaswell]

We should back out most of the changes from #16722

[anntzer]

Ah, I see, done.

[QuLogic]

Commit message needs updating.

[anntzer]

I guess I vaguely lost track of the deprecation status here. The diff says "remove the datapath" kwarg, but it's clearly not actually removed, just deprecated? so this just removes the ability to set it in the matplotlibrc. But perhaps in that case we may as well wait for the entire deprecation period to be elapsed and strip out the rcparam all at once?

[QuLogic]

So just wait for 3.4 then, and remove it all?

[anntzer]

I guess so, @tacaswell?

[tacaswell]

Re-milestoned to 3.4 as this is going to take some time to sort out and remind our selves what is going on and it is better to leave a deprecating thing sitting around warning for an extra cycle that it is to pull the rug out from under someone accidentally.

[QuLogic]

@tacaswell @anntzer can you figure out what needs to be done here?

[anntzer]

Are we fine with completely stripping out rcParams["datapath"] for 3.4? if so I can post a PR.

[tacaswell]

Suggested change

	- The ``datapath`` rcParam has been removed. Use `.get_data_path` instead.
	- The ``datapath`` rcParam is ignored. Use `.get_data_path` instead.

[tacaswell]

Yes to stripping out datapath for 3.4, we have given fair warning on this.

[anntzer]

Superseded by #19340.