Only install osxphotos if running on macOS about dogsheep-photos

[simonw]

https://www.python.org/dev/peps/pep-0508/#environment-markers I think I want sys_platform of darwin.

[simonw]

From https://hynek.me/articles/conditional-python-dependencies/ I think this will look like:

setup(
    # ...
    install_requires=[
        # ...
        "osxphotos>=0.28.13 ; sys_platform=='darwin'",
    ]
)

[RhetTbull]

FYI, I've got an issue to make osxphotos cross-platform but it's low on my priority list. About 90% of the functionality could be done cross-platform but right now the MacOS specific stuff is embedded throughout and would take some work. Though I try to minimize it, there's sprinklings of ObjC & Applescript throughout osxphotos.