Update pyknos wrt nflows

[michaeldeistler]

need to make a new PyPI version of pyknos such that nflows=0.12.0 is used.

(Installation still works, but gives an ugly error)

[MilesCranmer]

Maybe this will fix it? sbi-dev/pyknos#11
(and then a PyPI update)

[jan-matthis]

#289