New fstring failures

[felixonmars]

I'm not sure how to handle this one, so I did not submit a PR yet.

Source (from Python 3.7's dataclasses.py):
f"{f.name}={{self.{f.name}!r}}"

Parsed as:

JoinedStr(
    values=[
        FormattedValue(value=Attribute(value=Name(id='f'), attr='name'), conversion=-1, format_spec=None),
        Str(s='={self.'),
        FormattedValue(value=Attribute(value=Name(id='f'), attr='name'), conversion=-1, format_spec=None),
        Str(s='!r}')]),

which becomes a syntax error after rtrip:
f'{f.name}={self.{f.name}!r}'

  File "<fstring>", line 1
    (self.{f.name})
          ^
SyntaxError: invalid syntax