VaspInputSet couldn't write cif structures

[JiQi535]

Summary

This pr deals with the failure of writing .cif format structures by VaspInputSet. I encountered an error of 'PosixPath' object has no attribute 'lower' when using the write_input method with include_cif=True. After casting the PosixPath to str, this problem is solved.

[shyuep]

Pls add a test for this.

[coveralls]

Coverage remained the same at 0.0% when pulling 10afe19 on JiQi535:master into ffbebce on materialsproject:master.

[janosh]

@shyuep There is already a test for this actually.

pymatgen/pymatgen/io/vasp/tests/test_sets.py

Line 471 in ffbebce

self.mitset.write_input(".", make_dir_if_not_present=True, include_cif=True)

The tests are just not running because of

pymatgen/pymatgen/io/vasp/tests/test_sets.py

Lines 60 to 61 in ffbebce

	if SETTINGS.get("PMG_VASP_PSP_DIR") is None:
	raise unittest.SkipTest(f"PMG_VASP_PSP_DIR is not set. Skipping tests in {__file__}.")

We're not setting PMG_VASP_PSP_DIR. This is not due to #2708 btw, looks like they weren't running before either.

When I run

export PMG_VASP_PSP_DIR=/path/to/vasp/pseudo-potentials
pytest pymatgen/io/vasp/tests/test_sets.py -k test_write_input

I can reproduce the error @JiQi535 reported:

>       if fmt == "cif" or fnmatch(filename.lower(), "*.cif*"):
E       AttributeError: 'PosixPath' object has no attribute 'lower'