Support Python 3.14 unparenthesized exception lists

[Mola-maker]

Fixes #1319.

YAPF rejects Python 3.14 exception lists such as except ZeroDivisionError, ValueError, IndexError: because its vendored grammar permits at most one comma. Accept arbitrary unparenthesized exception lists, including trailing commas, for both except and except*, while keeping the as alternative limited to a single expression.

Keep a trailing exception-list comma adjacent to the clause's colon without changing existing slice spacing. Regression tests cover formatting and idempotence, long lists, trailing commas, existing parenthesized handlers, and rejection of unparenthesized lists with as.

Validation:

• python -m pytest -q: 615 passed on Python 3.8.20, 3.12.14, and 3.14.4 (Linux).
• Confirmed 10 new formatting subcases fail on the original code; the original suite has 611 passing tests.
• On Python 3.14, 12 additional cases preserve the AST, compile successfully, and remain unchanged on repeated formatting; the issue's reproducer also passes through the CLI.
• YAPF formatting, flake8 7.1.1, isort 6.0.0, and git diff --check pass for the changed files.

Prepared with Codex assistance.

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