Release 2.3 planning

[JukkaL]

This is the release tracking issue for 2.3, which I'm hoping to release on Monday Jul 13 on an accelerated schedule so that we'll have some recent mypyc features published before EuroPython (also I'll be on vacation after EuroPython for ~2.5 weeks). If any major regressions or unforeseen issues come up, we'll postpone the release date.

If there are any PRs you would like to ensure are included, please link them here.

[JukkaL]

Release branch created: https://github.com/python/mypy/pull/new/release-2.3

I'm going to cherry-pick mypyc improvements to the release branch.

[JukkaL]

Actually all the more recent non-mypyc master commitsare low-risk, so rebased the release branch on top of current master.

[JukkaL]

Cherry-pick:

• [mypyc] Make attribute access memory safe on free-threaded builds #21705

[ilevkivskyi]

@JukkaL Should we then close #21618 and instead include 2.2 regression fixes here instead of 2.2.1? (There is already one 2.2 regression fix in master)

[ilevkivskyi]

(also cc @hauntsaninja just in case)