In all of our current uses of os.umask or filesystem.umask, we always temporarily set it to a different value and then restore it using a try/finally block. I expect this pattern to continue.

Because of that, why don't we create a simple function in certbot.util that can be used as a context manager to set umask to a value and restore it to its previous value when exiting the with blo

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In order to address issue #3796, PR #3815 runs the e2e tests in series one after the other. It would be nice to run the tests in parallel again as that would lead to all the tests completing earlier.

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