Bump pymatgen from 2020.1.10 to 2020.1.28

Bumps [pymatgen](https://github.com/materialsproject/pymatgen) from 2020.1.10 to 2020.1.28.
- [Release notes](https://github.com/materialsproject/pymatgen/releases)
- [Changelog](https://github.com/materialsproject/pymatgen/blob/master/CHANGES.rst)
- [Commits](materialsproject/pymatgen@v2020.1.10...v2020.1.28)

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

requirements.txt

@@ -1,5 +1,5 @@
 pandas==0.25.3
-pymatgen==2020.1.10
+pymatgen==2020.1.28
 tensorflow==2.0.1
 keras==2.3.1
 scikit-learn==0.22.1