docs: Require sphinx-rtd-theme compatible with Sphinx 7

[matthewfeickert]

Description

• Update lower bound on sphinx-rtd-theme to v1.3.0rc1 to ensure compatibility with Sphinx v7+.
  • c.f. Add Sphinx 7 support readthedocs/sphinx_rtd_theme#1463
  • As soon as sphinx-rtd-theme v1.3.0 is released on PyPI the lower bound should be updated to reflect this stable release. The release candidate is being used to unbreak CI.
• Update lower bound of Sphinx to v7.0.0 for consistency.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Update lower bound on sphinx-rtd-theme to v1.3.0rc1 to ensure
  compatibility with Sphinx v7+.
   - c.f. https://github.com/readthedocs/sphinx_rtd_theme/issues/1463
   - As soon as sphinx-rtd-theme v1.3.0 is released on PyPI the lower
     bound should be updated to reflect this stable release. The release
     candidate is being used to unbreak CI.
* Update lower bound of Sphinx to v7.0.0 for consistency.

[matthewfeickert]

I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.