refix {{ roundtrip, fix hasattr static check

Author: wyattscarpenter

mypy/fastparse.py

@@ -173,8 +173,9 @@ def p() -> AST:
                 re.sub(r"#\s*mypy:\s*ignore(?![-_])", "# type: ignore", s)
                 if t == tokenize.COMMENT
                 else (
-                    s + "{"
+                    s[:-1]
                     if is_defective_version
+                    and hasattr(tokenize, "FSTRING_MIDDLE") #technically redundant as all the defective versions have this, but we'd like to appease the typechecker here
                     and t == tokenize.FSTRING_MIDDLE
                     and s.startswith("\\")
                     and s.endswith("{")