Fork failures are not uncommon on Cygwin, which causes an exception in the constructor.
Unfortunately, cleaning up the partially-initialized instance calls the `__del__` method, 
which then raises another exception because `self.pid` never got set.
This change should prevent the second exception.

Co-authored-by: Ronny Pfannschmidt <opensource@ronnypfannschmidt.de>

Forgot to put the `not` back in last time.

Make sure fork fails, then check that the destructor does not raise an exception and cleans up after itself.

I forgot the keyword for a keyword-only arg, then referenced a variable that would have been assigned after an exception was raised.
Add the keyword.
Remove variable that will never be assigned due to exception.

I think refcounting semantics would trigger the deconstructor call while exiting the context manager in CPython, but this makes the test less dependent on CPython implementation details.