Platform detection logic is flawed

[Jeremy Retailleau]

"win" is also in "darwin", so the installation is requiring "pywin32" on macOS
https://github.com/Preston-Landers/concurrent-log-handler/blob/master/setup.py#L61

A safer way to detect the platform would be:

if sys.platform.startswith("win"):
    ...

[Preston Landers]

Thanks for the PR!