Use kwarg for Theme filename

[bryevdv]

There was a recent change on Bokeh branch-2.4 to make the json and filename args to Theme be kwarg-only, but this is incompatible with Dask current usage. I will revert the change in Bokeh to maintain current compatibility, but I am also submitting the change here to set the stage for eventually re-introducing kwarg-only at some point in the future.

Secondarily, I have questions about what to do about testing? This was not caught by the "downstream" tests that Bokeh runs. A simple no-op test that just performs the import would have caught this since the use of Theme is at module scope. It's probably a good idea to make sure all of the bokeh-related modules cleanly import. But all of the bokeh related tests still seem to be in the dask repo, not this one. Should those tests be moved here? Or should I open a separate PR against dask to add tests?

Edit: reverted on Bokeh side at bokeh/bokeh#11484

[mrocklin]

+1 from me. I'm also happy bumping up the minimum version number higher if you would prefer.

[bryevdv]

I'm also happy bumping up the minimum version number higher if you would prefer.

I"m glad to hear this and I think I will have some improvements to memory and reference cycles in the very near future that will make bumping the min version even more compelling. But even so, in the immediate term I think making the forward-change here while delaying the change on the Bokeh side for a little while will avoid any version-related user pain.

I would definitely like to expand the bokeh-related testing regardless, to help keep on top of things. So any guidance about where those tests should live is appreciated.

[mrocklin]

Thanks @bryevdv ! This is in.