Fixes#456.

The relevant crash from the test log actually seems to be the initial failure:

AttributeError: module 'time' has no attribute 'clock_gettime_ns'The above exception was the direct cause of the following exception:...>       _time_machine.patch_if_needed()E       SystemError: <built-in function patch_if_needed> returned a result with an exception set

PyObject_GetAttrString fails to getclock_gettime andclock_gettime_ns on Windows because they only exist on Unix. It returnsNULL and each time sets theAttributeError as the current error.patch_if_needed did not clear this, leading to Python's call check to raise aSystemError caused by theAttributeError, making time-machine fail to start travelling.

I'm not sure which exact change in Python made this failure occur, so far it seems theSystemError should have been raised on older versions too. Ah well.

@gaborbernat I haven’t got a Windows setup for verifying this on - would you mind trying this on your test suite?