subprocess.Popen uses _winapi.CreateProcess() to spawn a child process. It stores the process handle using a Handle class. Popen never explicitly releases the handle, it rely on Handle.__del__ when the Popen object is detroyed (when Popen._handle attribute is cleared).

As discussed in bpo-37380, we could call explicitly CloseHandle(handle) as soon as the child process completes, to release resources in the Windows kernel.

Attached PR implements this fix.