[Maintenance] lift sphinx pin to <6

[psobolewskiPhD]

Closes #121

See the discussion there, it seems fine to lift now -- we already use sphinx_design.
See also: napari/napari#6239 (comment)

[melissawm]

Does this mean we should also change this here? https://github.com/napari/napari/blob/467d57b3967074370b2ad6750bdfc1af8aee67d6/resources/constraints/constraints_py3.10_docs.txt#L507

To be honest I'm not sure if this PR will do anything, since a new napari-sphinx-theme release would have to be made, and we are moving away from this anyway. But I'm getting errors locally when trying to build, I'm not sure why. Will investigate.

If I remember correctly, the pin was necessary for myst-parser, but that may not be true anymore. I'll check.

[Czaki]

Does this mean we should also change this here? napari/napari@467d57b/resources/constraints/constraints_py3.10_docs.txt#L507

This file is automatically updated every week (but could be triggered on need) base on metadata from packages on pypi.

So if a new release of napari-sphinx-theme will land on pypi then in week file should be upgraded. This file pins also napari-sphinx-theme version here https://github.com/napari/napari/blob/467d57b3967074370b2ad6750bdfc1af8aee67d6/resources/constraints/constraints_py3.10_docs.txt#L233