Consider Pex growing a hard dependency on psutil on Windows, or generally.

Since Pex for Windows is new, it's OK to do things like ship a platform-specific Pex PEX for Windows, etc.  Our doc server currently uses a very hacky shell-out to determine if a doc server is already running on Windows. This could stand using psutil or else the doc server will need to grow a ping endpoint and client side code to test that endpoint.