DEPS: unpin sphinx from doc build

[jreback]

once warnings are fixed in updated sphinx can unpin #38598

[jreback]

xref dask/dask#7001

seems 3.4.1 should fix.

[jorisvandenbossche]

It's probably not caused by that issue linked there, since the warnings we get is not for methods/attributes that are slots.
Looking a bit into it, and I think it's caused by sphinx-doc/sphinx#8533. We will have to see if this was intentional change or not on the sphinx side, but for the short term I put up a workaround: #38657