Update sphinx requirement from <6 to <8 in /requirements

[dependabot]

Updates the requirements on sphinx to permit the latest version.

Release notes

Sourced from sphinx's releases.

v7.0.0

Changelog: https://www.sphinx-doc.org/en/master/changes.html

Changelog

Sourced from sphinx's changelog.

Release 7.0.0 (released Apr 29, 2023)

Incompatible changes

• #11359: Remove long-deprecated aliases for MecabSplitter and DefaultSplitter in sphinx.search.ja.
• #11360: Remove deprecated make_old_id functions in domain object description classes.
• #11363: Remove the Setuptools integration (build_sphinx hook in setup.py).
• #11364: Remove deprecated sphinx.ext.napoleon.iterators module.
• #11365: Remove support for the jsdump format in sphinx.search.
• #11366: Make locale a required argument to sphinx.util.i18n.format_date().
• #11370: Remove deprecated sphinx.util.stemmer module.
• #11371: Remove deprecated sphinx.pycode.ast.parse() function.
• #11372: Remove deprecated sphinx.io.read_doc() function.
• #11373: Removed deprecated sphinx.util.get_matching_files() function.
• #11378: Remove deprecated sphinx.util.docutils.is_html5_writer_available() function.
• #11379: Make the env argument to Builder subclasses required.
• #11380: autosummary: Always emit grouped import exceptions.
• #11381: Remove deprecated style key for HTML templates.
• #11382: Remove deprecated sphinx.writers.latex.LaTeXTranslator.docclasses attribute.
• #11383: Remove deprecated sphinx.builders.html.html5_ready and sphinx.builders.html.HTMLTranslator attributes.
• #11385: Remove support for HTML 4 output.

Release 6.2.1 (released Apr 25, 2023)

Bugs fixed

• #11355: Revert the default type of :confval:nitpick_ignore and :confval:nitpick_ignore_regex to list.

Release 6.2.0 (released Apr 23, 2023)

Dependencies

• Require Docutils 0.18.1 or greater.

Incompatible changes

... (truncated)

Commits

• d568b2f Bump to 7.0.0 final
• ff79edf Remove jsdump references post removal
• 1a5133a Bump to 7.0.0rc1 final
• 5795fc7 Update sphinx.deprecation for Sphinx 7.0 (#11386)
• 6202087 Add a note to CHANGES for PR 11385
• ad47373 Remove HTML 4 support (#11385)
• 3e3251d Remove HTMLTranslator and html5_ready from sphinx.builders.html (...
• 77fd819 Remove deprecated LaTeXTranslator.docclasses attribute (#11382)
• 4be56f3 Remove deprecated style key for HTML templates (#11381)
• 49027a9 Autosummary: Always emit grouped ImportError exceptions (#11380)
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