Disable Process.pgid spec on Windows

[HertzDevil]

Process.pgid is not implemented on Windows and there is a pending spec for it. There are some Windows analogs to POSIX process group IDs:

• Console process groups control how processes receive Ctrl+C and Ctrl+Break interrupts. The process group ID is same as the process ID when CREATE_NEW_PROCESS_GROUP is given to CreateProcess, but it doesn't look like Win32 exposes a way to retrieve the ID itself for an arbitrary child process or whether this option was given to a process.
• Job objects can also be viewed as process groups. They are named Win32 objects so they technically have a string ID rather than a numeric one, but those names are for internal use only.
• Whatever Cygwin does.

It is unlikely there will ever be a suitable analog on Windows. This PR permanently disables this pending spec on Windows.