Bump pandas from 1.2.5 to 1.3.1 in /extra_requirements (#9)

Bumps [pandas](https://github.com/pandas-dev/pandas) from 1.2.5 to 1.3.1.
- [Release notes](https://github.com/pandas-dev/pandas/releases)
- [Changelog](https://github.com/pandas-dev/pandas/blob/master/RELEASE.md)
- [Commits](pandas-dev/pandas@v1.2.5...v1.3.1)

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

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

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

extra_requirements/requirements-tests.txt

@@ -17,7 +17,7 @@ multihist==0.6.4
 npshmex==0.2.1                                  # Strax dependency
 numba==0.53.1                                   # Strax dependency
 numpy==1.19.5
-pandas==1.2.5                                   # Strax dependency
+pandas==1.3.1                                   # Strax dependency
 psutil==5.8.0                                   # Strax dependency
 pytest==6.2.4
 pytest-cov==2.12.1