python / mypy_extensions

Bump version to 0.4.3.

Bump version to 0.4.2.

Release 0.4.1
This is again a retroactive artifact of the repo move.
This differs from 0.4.0 literally only by the version number.

Presumbably there was a miscommunication about what we had released?

Release 0.4.0
This release was of course actually done a long time ago,
but I am redoing it in the new repo.