Fix test on 3.9

Author: JukkaL

mypyc/test-data/run-classes.test

@@ -3796,6 +3796,8 @@ assert t.bitfield == 0x0C
 assert t.default == 10
 
 [case testPerTypeFreeList]
+from __future__ import annotations
+
 from mypy_extensions import mypyc_attr
 
 a = []