[Distutils] What is the syntax for passing conditional non-extra dependencies in setuptools?

[Daniel Holth]

How do I specify a conditional (marker-guarded) non-extra dependency
in setuptools? The syntax for a conditional extra dependency is
currently:

    extras_require = {
        "ssl:sys_platform=='win32'": "wincertstore==0.2",
        "certs": "certifi==1.0.1",
    },