Fix _TupleProxy docstring

Author: clayote

src/attr/_make.py

@@ -104,13 +104,13 @@ def __reduce__(self, _none_constructor=type(None), _args=()):  # noqa: B008
 
 
 class _TupleProxy(Sequence):
-    __slots__ = ("_tup",)
     """A wrapper for a tuple that makes it not type-check as a tuple
     
     This is a hack to make Sphinx document all cached properties on slots
     classes as if they were regular properties.
     
     """
+    __slots__ = ("_tup",)
 
     def __init__(self, tup: tuple):
         self._tup = tup