[issue27921] f-strings: do not allow backslashes

[Roundup Robot]

Roundup Robot added the comment:

New changeset fd4e4fa75260 by Eric V. Smith in branch 'default':
Issue 27921: Remove backslash from another f-string. I'll revert this change before beta 2. I also need to look in to why test_tools/test_unparse fails with the files that are now being skipped.
https://hg.python.org/cpython/rev/fd4e4fa75260

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