MAINT: bump upper sphinx version limit to 5.1

[michaelosthege]

Since 0.5.0 this package should be compatible with Sphinx 5, or more precisely with sphinx<5.1 as because of #101.

As explained in https://github.com/conda-forge/sphinx-jupyterbook-latex-feedstock/pull/11#issue-1435774378 I'm using this successfully with Sphinx 5.0.1, but conda/mamba currently don't resolve such environments because sphinx-jupyterbook-latex still specifies sphinx<5.

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Codecov Report

Merging #114 (f91555d) into master (d841b28) will not change coverage. The diff coverage is n/a.

Additional details and impacted files

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