No compatibility with pipenv

This problem was first issued under `pipenv` repo ([pipenv/issues/1575](https://github.com/pypa/pipenv/issues/1575)) but as it turned out it was not `pipenv` that was at fault.



##### Problem description:

If we place `pywin32` (or `pypiwin32`) in `Pipfile` for example like this:

```

[packages]

"pywin32" = {version = "*", sys_platform = "== 'win32'"}

```

and then we do `pipenv install` on **Windows** everything will be fine. But if we do the same under **Linux** then we are in bad luck. Even though pipenv is not installing this package (because we specified to install it only under windows) we will still get an error:

```

Warning: Your dependencies could not be resolved. You likely have a mismatch in your sub-dependencies.

  You can use $ pipenv install --skip-lock to bypass this mechanism, then run $ pipenv graph to inspect the situation.

Could not find a version that matches pywin32>=223

Tried: (no version found at all)

```



For the matter other system-specific packages like `pywinusb` or `stdeb` work fine with pipenv in identical scenario.



As stated by `pipenv` contributors:

> (...) this is not an issue with pipenv, it's an issue with how pywin32 has organized their setup.py, we literally have no way to work around this and it is only a problem for this specific package.

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