Fix Python 3.9-incompatible union syntax

Author: clayote

src/attr/_make.py

@@ -105,11 +105,12 @@ def __reduce__(self, _none_constructor=type(None), _args=()):  # noqa: B008
 
 class _TupleProxy(Sequence):
     """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):

src/attrs/sphinx_cached_property.py

@@ -6,6 +6,9 @@
 classes will be inaccessible.
 
 """
+
+from __future__ import annotations
+
 from functools import cached_property
 from typing import Any
 

tests/doctest_data/explicit-autoproperty-cached.rst

@@ -1,3 +1,3 @@
 .. autoclass:: tests.test_slots.SphinxDocTest
 
-    .. autoproperty:: documented
+    .. autoproperty:: documented

tests/doctest_data/members-cached-property.rst

@@ -1,2 +1,2 @@
 .. autoclass:: tests.test_slots.SphinxDocTest
-    :members:
+    :members:

tests/test_slots.py

@@ -8,24 +8,22 @@
 import pickle
 import shutil
 import weakref
-from itertools import zip_longest
 
+from itertools import zip_longest
 from pathlib import Path
 from unittest import mock
 
 import hypothesis.strategies as st
 import pytest
-from hypothesis import given
-
-from attr._make import _TupleProxy
-
 
+from hypothesis import given
 from sphinx.application import Sphinx
 
 import attr
 import attrs
 
 from attr._compat import PY_3_14_PLUS, PYPY
+from attr._make import _TupleProxy
 
 
 # Pympler doesn't work on PyPy.