Disable a test that fails in Py2 in 0.29.x since 'str' is 'unicode' in cython3, also in Python 2.

tests/run/cython3.pyx

@@ -632,9 +632,11 @@ async def async_def_annotations(x: 'int') -> 'float':
     return float(x)
 
 
+'''
 def repr_returns_str(x) -> str:
     """
     >>> repr_returns_str(123)
     '123'
     """
     return repr(x)
+'''