[issue28978] a redundant right parentheses in the EBNF rules of parameter_list

[Martin Panter]

Changes by Martin Panter <vadmium+py at gmail.com>:


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stage:  -> needs patch
versions: +Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28978>
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