bpo-28002: Roundtrip f-strings with ast.unparse better (#19612)

By attempting to avoid backslashes in f-string expressions.
We also now proactively raise errors for some backslashes we can't
avoid while unparsing FormattedValues

Co-authored-by: hauntsaninja <>
Co-authored-by: Shantanu <hauntsaninja@users.noreply.github.com>
Co-authored-by: Batuhan Taskaya <isidentical@gmail.com>