`SelectorEventLoop` on Windows does not support subprocesses

[hendrikmakait]

Due to an old issue with tornado<6.1, we are currently using the SelectorEventLoop on Windows. However, that event loop does not support subprocesses. This causes the SubprocessCluster in #7431 to not support Windows.

Upgrading to tornado>=6.1 should allow us to use the default ProActorEventLoop (see #5833 for a dormant PR on this).

[jrbourbeau]

Thanks @hendrikmakait! I think not having support for Windows in SubprocessCluster initially is okay