[syntax-errors] Parenthesized context managers before Python 3.9

[ntBre]

Summary

I thought this was very complicated based on the comment here: #16106 (comment) and on some of the discussion in the CPython issue here: python/cpython#56991. However, after a little bit of experimentation, I think it boils down to this example:

with (x as y): ...

The issue is parentheses around a with item with an optional_var, as we (and Python) call the trailing variable name (y in this case). It's not actually about line breaks after all, except that line breaks are allowed in parenthesized expressions, which explains the validity of cases like

>>> with (
...     x,
...     y
... ) as foo:
...     pass
... 

even on Python 3.8.

I followed pyright's example again here on the diagnostic range (just the opening paren) and the wording of the error.

Test Plan

Inline tests

[github-actions]

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[MichaReiser]

Do we also need to emit the same diagnostic on line 2144, e.g. for with (foo, bar):

[MichaReiser]

It would be nice to add documentation, similar to what you did for other variants.