TaskGroup: Explicitly None check for propagated cancellation error

[matan1008]

The issue is trivial IMHO, if you think otherwise I will submit an issue.

[bedevere-app]

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[gvanrossum]

How did you discover this? Since it's always either None or a CancelledError, which doesn't override __eq__ or __bool__, the original code should be safe, right?

[matan1008]

Technically (and stupidly practically) someone might raise an exception from the following kind inside the TaskGroup code block:

class StrangeException(asyncio.CancelledError):
    def __bool__(self):
        return False

Which in my humble opinion might result in unexpected behaviour