Bump sphinx from 4.3.2 to 4.4.0 (AtsushiSakai#625)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 4.3.2 to 4.4.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v4.3.2...v4.4.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

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

Author: dependabot[bot]

requirements.txt

@@ -7,6 +7,6 @@ pytest == 6.2.5 # For unit test
 pytest-xdist == 2.5.0 # For unit test
 mypy == 0.931 # For unit test
 flake8 == 4.0.1 # For unit test
-sphinx == 4.3.2 # For sphinx documentation
+sphinx == 4.4.0 # For sphinx documentation
 sphinx_rtd_theme == 1.0.0
 IPython == 8.0.0 # For sphinx documentation