build(deps): bump pymatgen in /mpcontribs-client
build(deps): bump pymatgen in /mpcontribs-client

Bumps [pymatgen](https://github.com/materialsproject/pymatgen) from 2020.8.13 to 2020.9.14.
- [Release notes](https://github.com/materialsproject/pymatgen/releases)
- [Changelog](https://github.com/materialsproject/pymatgen/blob/master/CHANGES.rst)
- [Commits](https://github.com/materialsproject/pymatgen/compare/v2020.8.13...v2020.9.14)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
materialsproject/MPContribs,materialsproject/MPContribs,materialsproject/MPContribs,materialsproject/MPContribs