Update sphinx requirement from <4.0 to <5.0 (AtsushiSakai#612)

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/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v4.3.2)

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

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

docs/requirements.txt

@@ -1,3 +1,3 @@
-sphinx < 4.0 # For sphinx documentation
+sphinx < 5.0 # For sphinx documentation
 sphinx_rtd_theme == 1.0.0
 IPython == 7.30.1 # For sphinx documentation

requirements.txt

@@ -5,6 +5,6 @@ matplotlib == 3.5.1
 cvxpy == 1.1.17
 pytest == 6.2.5 # For unit test
 pytest-xdist == 2.5.0 # For unit test
-sphinx < 4.0 # For sphinx documentation
+sphinx < 5.0 # For sphinx documentation
 sphinx_rtd_theme == 1.0.0
 IPython == 7.30.1 # For sphinx documentation