fix: uproot was exposed in one place to dask's _task_spec overhaul

[lgray]

c.f. dask-contrib/dask-awkward#556

There don't appear to be any other places where uproot directly accesses dask innards.

[lgray]

@jpivarski please merge this as soon as it's all passing, we will need a timely release as well.

[lgray]

ah - of course this waits on the dask-awkward release first :-p

[pfackeldey]

@lgray there's no reason to increase the dask-awkward version requirement, or is there? The change checks for an update in dask for the new task_spec, but still handles the old case. (I think this PR is independent of the dask-awkward PR aswell in this sense)

edit: oh well, ok, there is a need for this, because without there could be a state where dask uses new task_spec, uproot can handle it (with this PR), but dask-awkward doesn't. Nevermind then, sorry for the noise!

[lgray]

And it's only for the dev extra as well, and since we check back to py39 we test all code paths.

[jpivarski]

I see that this follows upstream changes in Dask and has Uproot taking a new minimum dask-awkward version.

Looks good to me! I'll make a new release right after merging this PR.