An unexpected behavior lookup:

```python
>>> ast.FormattedValue(ast.Str('ss')).conversion
>>> ast.unparse(ast.FormattedValue(ast.Str('ss')))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/ast.py", line 1564, in unparse
    return unparser.visit(ast_obj)
  File "/usr/local/lib/python3.9/ast.py", line 801, in visit
    self.traverse(node)
  File "/usr/local/lib/python3.9/ast.py", line 795, in traverse
    super().visit(node)
  File "/usr/local/lib/python3.9/ast.py", line 407, in visit
    return visitor(node)
  File "/usr/local/lib/python3.9/ast.py", line 1153, in visit_FormattedValue
    self._fstring_FormattedValue(node, self.buffer_writer)
  File "/usr/local/lib/python3.9/ast.py", line 1178, in _fstring_FormattedValue
    conversion = chr(node.conversion)
TypeError: an integer is required (got type NoneType)
>>> ast.unparse(ast.FormattedValue(ast.Str('ss'), -1))
'f"{\'ss\'}"'
```

ast.FormattedValue conversion's default value is expected to be -1 but not None

See: https://docs.python.org/3/library/ast.html#ast.FormattedValue

Other:

If certainly, it's also a bug on typeshed.