Release 3.29.3: changelog, version and documentation

[absurdfarce]

Pretty much what it says on the tin. Add changelog entries and update the version string for the new release. Also fix any version-specific doc links.

[absurdfarce]

@bschoening I think this is the last thing we need for 3.29.3.

This is the 3.29.3 equivalent to this commit. After this guy goes in we can tag the corresponding commit as 3.29.3 and push to pypi. There may be some follow-up work from that point (such as this commit which updates the internal doc publishing links to point to the new tag) but that requires a good 3.29.3 tag... so it makes sense to do after the release.

[bschoening]

@absurdfarce LGTM.

Next release, 3.30.x, should support Python 3.14 and EOL 3.9.

[absurdfarce]

Agreed re: 3.30.0 support. Ideally 3.29.3 would've supported 3.14 but I didn't want to delay this release by working on a new version... after all this was supposed to be a quick turnaround kind of thing. And adding 3.14 support would be a pretty good thing to aim at for our next minor release (along with the 3.12 reactor fixes).