Define dependency in pyproject.toml depending on operating system

Question

I am using pyproject.toml according to PEP 631 (i.e. NO poetry, I am using pdm). I need to specify a dependency version according to the operating system.

So far, I tried something like this:

[project]
...
dependencies = [
    "kaleido==0.0.3; platform_system!='Windows'",
    "kaleido==0.1.0post1; platform_system=='Windows'",
]

What I want to achieve is that kaleido will be installed in version 0.0.3 if the operating system is NOT Windows. On the other hand, if Windows is used, then I want kaleido to be installed in version 0.1.0post1.

What I learned is that the first row will always be ignored, that is, if I am installing the package on a linux machine, kaleido v0.0.3 won't be installed. On a Windows machine kaleido v0.1.0post1 will successfully be installed.

If I switch the two code lines, the behaviour will be the opposite, i.e. kaleido v0.0.3 will be installed on the linux machine while no kaleido will be installed on a Windows machine.

The actual question is, what is the correct syntax in the pyproject.toml to work on all operating systems?