```
In [4]: ast.dump(ast.parse('f"{x!r}"'))
Out[4]: "Module(body=[Expr(value=JoinedStr(values=[FormattedValue(value=Name(id='x', ctx=Load()), conversion=114)]))], type_ignores=[])"
In [5]: ast.dump(ast.parse('f"{x}"'))
Out[5]: "Module(body=[Expr(value=JoinedStr(values=[FormattedValue(value=Name(id='x', ctx=Load()), conversion=-1)]))], type_ignores=[])"
```
(On 3.9 but I don't think this has changed since 3.6.)
The stdlib also assumes this: https://github.com/python/cpython/blob/main/Lib/ast.py#L1211 (`chr(None)` doesn't work).
This attribute was [added in 3.8]. This change lets mypy resolve the
type correctly instead of failing with an attr-defined error.
[added in 3.8]: https://github.com/python/cpython/pull/13479
