[issue28489] Fix comment in tokenizer.c

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

There are just 8 legal combinations if ignore case:

>>> import tokenize
>>> sorted({x.lower() for x in tokenize._all_string_prefixes() if x})
['b', 'br', 'f', 'fr', 'r', 'rb', 'rf', 'u']

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nosy: +serhiy.storchaka

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<http://bugs.python.org/issue28489>
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