Fix `intersphinx` for the `cosmology` package

[paddyroddy]

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Following #347 we now have static types rendering in the documentation. However, as can be seen in the screenshot, npt.NDArray isn't highlighted. Anyone know if it's possible?

[Saransh-cpp]

intersphinx does this, but we will need to import numpy.typing in every file without the TYPE_CHECKING condition (sphinx-doc/sphinx#10746), or figure out a way to do that on the fly by writing a custom extension (see https://github.com/pybamm-team/PyBaMM/tree/develop/docs/sphinxext for instance).

[Saransh-cpp]

Given the files have a "non-conditional" import of numpy.typing, some of the docs render (ugly imo) cross-references to numpy types -

Should we should stick to one thing? Either import numpy.typing without a condition or with. If we want to add the condition, we can move the types declared in this file to a utility file (_typeutils.py or something).

Originally posted by @Saransh-cpp in #347 (comment)

[paddyroddy]

So looking into this, it has nothing to do with TYPE_CHECKING. It's look alright as of #381. However, I'm trying to work out how to get it working for the cosmology package, which doesn't seem to work out the box.

[paddyroddy]

So looks like everything is working except https://cosmology.readthedocs.io/en/stable. I tried both

intersphinx_mapping = {
    "cosmology": ("https://cosmology.readthedocs.io/en/stable", None),
    "numpy": ("https://numpy.org/doc/stable", None),
    "python": ("https://docs.python.org/3", None),
}

and

intersphinx_mapping = {
    "cosmology": (
        "https://cosmology.readthedocs.io/en/stable",
        "https://cosmology.readthedocs.io/en/stable/objects.inv",
    ),
    "numpy": ("https://numpy.org/doc/stable", None),
    "python": ("https://docs.python.org/3", None),
}

any ideas? Neither worked. Won't matter if #124 is tackled first.