Hide all type-hint-only imports behind if TYPE_CHECKING

[janosh]

4 main motivations for this change:

1. reduces chance of circular imports
2. can help contributors run tests without installing extra deps like pymatgen.analysis.defects if they were previously in a test's import stack but only used for type hints
3. helps with readability by clearly separating type-only imports
4. can decrease startup time since TYPE_CHECKING=False at run time means some modules won't have to be imported. somewhat negated by the annoyingly long import time of the std lib typing module itself but that has been steadily coming down with newer Python versions.

[janosh]

Ci failure is due to flaky test:

    @unittest.skipIf(
        not SETTINGS.get("PMG_MAPI_KEY") or website_down,
        "PMG_MAPI_KEY environment variable not set or MP is down.",
    )
    def test_get_snls_mp(self):
        with OptimadeRester("mp") as optimade:
            structs = optimade.get_snls(elements=["Ga", "N"], nelements=2)
    
        with OptimadeRester("mp") as optimade:
            response_field_structs_single = optimade.get_snls(
                elements=["Ga", "N"], nelements=2, additional_response_fields="nsites"
            )
            response_field_structs_set = optimade.get_snls(
                elements=["Ga", "N"], nelements=2, additional_response_fields={"nsites", "nelements"}
            )
            if ("mp" in response_field_structs_single) and ("mp" in response_field_structs_set):
>               assert len(structs["mp"]) == len(response_field_structs_single["mp"])
E               KeyError: 'mp'

pymatgen/ext/tests/test_optimade.py:55: KeyError