Update PyPi minor release
The last minor NIFTy8 release on PyPi (https://pypi.org/project/nifty8/#history) is 8.4 (from 29.9.22). Since many commits have been merged in between (including many bugfixs), it would be good to release a new minor stable version to keep the PiPy version more or less up to date. @pfrank @gedenhof @mtr