Bump pytest-xdist from 3.3.0 to 3.3.1 in /requirements (AtsushiSakai#838)

Bumps [pytest-xdist](https://github.com/pytest-dev/pytest-xdist) from 3.3.0 to 3.3.1.
- [Changelog](https://github.com/pytest-dev/pytest-xdist/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-xdist@v3.3.0...v3.3.1)

---
updated-dependencies:
- dependency-name: pytest-xdist
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

requirements/requirements.txt

@@ -3,6 +3,6 @@ scipy == 1.10.1
 matplotlib == 3.7.1
 cvxpy == 1.3.1
 pytest == 7.3.1 # For unit test
-pytest-xdist == 3.3.0 # For unit test
+pytest-xdist == 3.3.1 # For unit test
 mypy == 1.3.0 # For unit test
 ruff == 0.0.267 # For unit test