Enable PyBetter's FixTrivialNestedWiths when Black improves multiple context manager formatting

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psf/black#664 proposes breaking very long with-statements using backslashes (< 3.9) or parentheses (>= 3.9), and shows the ugly current formatting for such statements.

Once that's done, re-enabling FixTrivialNestedWiths will be an obvious win. If Black initially only supports parenthesised context managers we can re-enable this refactoring for py39+ only.

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Thanks to psf/black#3489, this now works on Python 3.9 and later. I propose that shed --refactor should follow suit.

Python 3.7 will reach end of life before psf/black#3484 ships (black == 24.1.0 at earliest), but we can enable this feature for 3.8 if Black ships it before 3.8 reaches EOL too.