Update pymatgen requirement

Updates the requirements on [pymatgen](https://github.com/materialsproject/pymatgen) to permit the latest version.
- [Release notes](https://github.com/materialsproject/pymatgen/releases)
- [Changelog](https://github.com/materialsproject/pymatgen/blob/master/docs/CHANGES.md)
- [Commits](materialsproject/pymatgen@v2019.7.2...v2023.12.18)

---
updated-dependencies:
- dependency-name: pymatgen
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -1,6 +1,6 @@
 optimade[server]~=0.25.3
 aiida-core~=2.4.0
-pymatgen>=2019.7.2,!=2019.9.7,<2023.11.13
+pymatgen>=2019.7.2,!=2019.9.7,<2023.12.19
 
 # Dependencies used directly in this package, but included through other dependencies:
 # aiida-core: