> 4) use Job objects to group Windows processes for termination

I think a separate issue should be created for this enhancement.

_winapi wrappers would be needed for CreateJobObjectW(), SetInformationJobObject(), AssignProcessToJobObject(), TerminatejobObject(), and ResumeThread(), plus the constant CREATE_SUSPENDED. I'd also prefer to make related changes to send_signal(), which would require GetConsoleProcessList() and GenerateConsoleCtrlEvent().

A new Popen option would be needed to configure whether the job allows descendants to break away via the process creation flag CREATE_BREAKAWAY_FROM_JOB. This should be allowed by default.

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send_signal(): SIGKILL, SIGTERM, SIBREAK, SIGINT

Support Popen.kill(group=False) and Popen.terminate(group=False) on all platforms as Popen.send_signal(signal.SIGKILL, group=group) and Popen.send_signal(signal.SIGTERM, group=group).

The Universal CRT (ucrt) in Windows doesn't define SIGKILL. Even when it's not defined by the platform, signal.SIGKILL should always be defined, preferably as 9 (unused by ucrt), else as an unused value in the range up to NSIG, else as NSIG + 1.

The `group` keyword-only option broadens the scope to the process group or job. A process is a group leader if it was created with the flag CREATE_NEW_PROCESS_GROUP (save self._creationflags) and its process ID is in GetConsoleProcessList().

For SIGKILL, always use forced termination. For SIGTERM, use forced termination either if `group` is false or if `group` is true and the process is not a group leader. To force termination, call TerminateJobObject(self._job_handle, 1) if `group` is true, else TerminateProcess(self._handle, 1). 

For SIGTERM, SIGBREAK, and SIGINT, call GenerateConsoleCtrlEvent() if `group` is true and the process is a group leader. For SIGTERM and SIGBREAK, send CTRL_BREAK_EVENT. For SIGINT, send CTRL_C_EVENT. 

Behavior to deprecate:

When `group` is false and `sig` is CTRL_C_EVENT (0) or CTRL_BREAK_EVENT (1), send_signal() always calls os.kill(). This legacy behavior tries to handle a process as if it's a process group, and it combines POSIX kill() with Windows API constants. subprocess should distance itself from the fundamental problems with os.kill() in Windows.