Use casts to fix typechecking of visitor method

-> should we use a more complex solution by building an ast expression of
`str(...)` to get rid of the casts?

Author: Adrien Chauve

mypy/fastparse.py

@@ -776,7 +776,8 @@ def visit_Str(self, n: ast35.Str) -> Union[UnicodeExpr, StrExpr]:
         def visit_JoinedStr(self, n: ast35.JoinedStr) -> StrExpr:
             result_string_expression = StrExpr('')
             for value in n.values:
-                result_string_expression = OpExpr('+', result_string_expression, self.visit(value))
+                value_as_string_expr = cast(StrExpr, self.visit(value))
+                result_string_expression = cast(StrExpr, OpExpr('+', result_string_expression, value_as_string_expr))
             return result_string_expression
 
         # FormattedValue(expr value)