[issue27921] f-strings: do not allow backslashes

[Roundup Robot]

Roundup Robot added the comment:

New changeset df5b6d1c7ea8 by Benjamin Peterson in branch 'default':
fix skipping #27921 for windows
https://hg.python.org/cpython/rev/df5b6d1c7ea8

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27921>
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