DOC: Set 0.25.2 in index.rst

[TomAugspurger]

Forgot this for 0.25.1

[WillAyd]

Have you historically just done this whenever creating a new whatsnew release (or around that time)? Loosely wondering if there's a better way to automate this, though low priority I would think

[TomAugspurger]

Yeah, this can / should be done when the new doc/source/whatsnew/v... is created. Automating is a bit tricky since we don't necessarily know whether we're doing another release on that branch though.

A small script like python scripts/start_release 0.25.3 that

1. Makes the right whatsnew
2. Populates the whatsnew with the empty sections
3. Adds to whatsnew/index.rst
4. Updates index.rst.template

would be useful, but somewhat tricky to get right.

[WillAyd]

Hmm was this supposed to trigger a back port? Didn't see one come through