Update sphinx requirement from ^4.3.2 to >=4.3.2,<7.0.0 (#40)

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v4.3.2...v6.2.1)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

pyproject.toml

@@ -68,7 +68,7 @@ pyupgrade = ">=2.29,<4.0"
 optional = true
 
 [tool.poetry.group.docs.dependencies]
-sphinx  = "^4.3.2"
+sphinx  = ">=4.3.2,<7.0.0"
 sphinx_book_theme = ">= 1.0.1, < 2.0"
 sphinx-copybutton = "*"
 docutils = "*"