Update sphinx requirement from <6,>=3.0.0 to >=3.0.0,<7

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@v3.0.0...v6.0.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

setup.py

@@ -33,7 +33,7 @@ def _get_long_description():
         'scipy >=1.0.0, <2',
     ],
     extras_require={
-        'docs': ['sphinx >=3.0.0, <6', 'matplotlib >=3.1', 'numpydoc', 'm2r2'],
+        'docs': ['sphinx >=3.0.0, <7', 'matplotlib >=3.1', 'numpydoc', 'm2r2'],
         'tests': ['pytest', 'coverage <8', 'pytest-cov', 'coveralls'],
     },
     package_data={"csaps": ["py.typed"]},