bump pymatgen in /mpcontribs-sidecars/kernel_gateway

Bumps [pymatgen](https://github.com/materialsproject/pymatgen) from 2020.12.31 to 2021.2.8.1.
- [Release notes](https://github.com/materialsproject/pymatgen/releases)
- [Changelog](https://github.com/materialsproject/pymatgen/blob/master/CHANGES.rst)
- [Commits](https://github.com/materialsproject/pymatgen/commits)

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

mpcontribs-sidecars/kernel_gateway/requirements.txt

@@ -3,4 +3,4 @@ jupyter-kernel-gateway==2.4.3
 mpcontribs-client==3.8.1
 plotly==4.14.3
 psutil==5.8.0
-pymatgen==2020.12.31
+pymatgen==2021.2.8.1