Windows. Process.terminate() may fail with ERROR_INVALID_HANDLE

[giampaolo]

Acknoledgement of this originated from the following Windows failure re. Python subprocess module:
https://ci.appveyor.com/project/giampaolo/psutil/builds/22546914/job/rlp112gffyf2o30i
https://bugs.python.org/issue36067

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ERROR: psutil.tests.test_process.TestProcess.test_halfway_terminated_process
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Traceback (most recent call last):
  File "c:\projects\psutil\psutil\tests\test_process.py", line 85, in tearDown
    reap_children()
  File "c:\projects\psutil\psutil\tests\__init__.py", line 493, in reap_children
    subp.terminate()
  File "C:\Python35-x64\lib\subprocess.py", line 1092, in terminate
    _winapi.TerminateProcess(self._handle, 1)
OSError: [WinError 6] The handle is invalid

psutil currently does not catch ERROR_INVALID_HANDLE so basically it has the same issue. This is hard to replicate because the process should disappear between OpenProcess and TerminateProcess calls, see here. Still, we know from subprocess module that the risk is non-zero.