[dateutil] Check platform before imports

[eliroca]

Otherwise imports may fail on other platforms.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[brianschubert]

Coming from #15574, as I understand it, this was a problem when checking the stubs with mypy 1.8.0, which doesn't include ctypes._NewTypes in its vendored copy of the stdlib stubs.

With current mypy / typeshed, these imports exist on all platforms, so I don't think this is necessary here.

For what it's worth, I'm not sure this helps with your use case either, since _NewTypes won't be defined if you ever check the stubs on Windows. In your forked stubs, it would probably make sense to either add a local copy of _NewTypes or to delete the win.pyi stubs altogether, depending on whether you intend to support Windows or not.

I'll go ahead and close this one, but thanks for the PR in any case! If you do find any other issues with the dateutil stubs, PRs are always welcome!