Use typing_extensions only if needed

This makes the typing-extensions test dependency < 3.11 only, and
conditionally imports assert_type from typing or typing_extensions
depending on the Python version.

Author: EliahKagan

test-requirements.txt

@@ -8,4 +8,4 @@ pytest-cov
 pytest-instafail
 pytest-mock
 pytest-sugar
-typing-extensions
+typing-extensions ; python_version < "3.11"

test/deprecation/test_attributes.py

@@ -9,8 +9,14 @@
 """
 
 import itertools
+import sys
 from typing import Type
 
+if sys.version_info >= (3, 11):
+    from typing import assert_type
+else:
+    from typing_extensions import assert_type
+
 import pytest
 from typing_extensions import assert_type