fix(docs): fix {cls} formatting (#322)

a_sync/iter.py

@@ -108,7 +108,8 @@ def __init_subclass__(cls, **kwargs) -> None:
             cls.__doc__ += f"\n\n{new}"
 
         # format the member docstrings
-        for attr in cls.__dict__.values():
+        for attr_name in dir(cls):
+            attr = getattr(cls, attr_name)
             if attr.__doc__ and "{cls}" in attr.__doc__:
                 attr.__doc__ = attr.__doc__.replace("{cls}", f":class:`{cls.__name__}`")