Bump pandas from 1.3.5 to 1.4.0 in /requirements (AtsushiSakai#634)

Bumps [pandas](https://github.com/pandas-dev/pandas) from 1.3.5 to 1.4.0.
- [Release notes](https://github.com/pandas-dev/pandas/releases)
- [Changelog](https://github.com/pandas-dev/pandas/blob/main/RELEASE.md)
- [Commits](pandas-dev/pandas@v1.3.5...v1.4.0)

---
updated-dependencies:
- dependency-name: pandas
  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/requirements.txt

@@ -1,6 +1,6 @@
 numpy == 1.22.1
 scipy == 1.7.3
-pandas == 1.3.5
+pandas == 1.4.0
 matplotlib == 3.5.1
 cvxpy == 1.1.18
 pytest == 6.2.5 # For unit test