greentest/mysubprocess.py: add interrupt() method, make send_signal() not raise ValueError if signal not available and use taskkill on windows if otherwise killing does not work

use Event instead of switch()

socket: fix 2.7 issue where buffer is no longer accepted as sendall's argument. using memoryview instead. Thanks to Luigi Pugnetti.

patched tests: do not report ModuleNotFoundWarning for the tests that are not present in a particular version of Python