Add stubs for OpenEXR and OpenImageIO

[chadrik]

The signatures provided by pyblind11 are not as accurate as we'd like them to be. For example:

>>> import OpenImageIO
>>> print(OpenImageIO.TextureSystem.imagespec.__doc__)
imagespec(self: OpenImageIO.OpenImageIO.TextureSystem, filename: str, subimage: int = 0) -> object

The wrapper for this function looks like this:

       .def(
            "imagespec",
            [](TextureSystemWrap& ts, const std::string& filename,
               int subimage) -> py::object {
                py::gil_scoped_release gil;
                const ImageSpec* spec
                    = ts.m_texsys->imagespec(ustring(filename), subimage);
                if (!spec) {
                    return py::none();
                }
                return py::object(py::cast(*spec));
            },
            "filename"_a, "subimage"_a = 0)

The type py::object is used because the result type varies. From looking at the wrapper code above, we know that true return type of this function, in python terms, is actually typing.Optional[ImageSpec] (which in more recent versions of python can be written as ImageSpec | None).

The manual fixes that I've had to make fall into a few categories:

• Buffer should be numpy.ndaarray: This refers to the new typing.Buffer protocol, and I just read that this is officially supported through pybind11 using py::buffer. Since we know that Buffer is the name emitted by pybind11, I can make a PR against mypy to support this. However, in the short term I think we need to keep annotating this as numpy.ndarray because IIUC the buffer protocol is only supported in python 3.12 and later.
• list[X] should be Iterable[X]. Honestly, this is an issue with pybind11’s docstring generation, because it seems to permit any kind of iterable instead of just a list.
• TypeDesc should be Union[TypeDesc, BASETYPE, str]: Is this handled using implicit conversion? If so, it seems reasonable that pybind11 should produce a better annotation for this type, as it should know the alternatives.
• Optional results: this is the majority of the issues, and the thorniest. How can we make pybind11 know when a type is X or None?
• histogram and isConstantColor should be tuple types. Would this help? https://pybind11.readthedocs.io/en/stable/advanced/misc.html#setting-inner-type-hints-in-docstrings