Add typing-extensions to test deps. for Python <3.9

Add `typing-extensions` to `requirements-tests.txt` since
`tests/test_main.py` imports it; but at the same time, since `typing` in
the Python standard library provides both `Annotated` and
`get_type_hints`, adapt the tests to use the standard library instead on
Python 3.9 and later.

Author: musicinmybrain

requirements-tests.txt

@@ -1,6 +1,7 @@
 -e .
 
 pytest >=8.3.5
+typing-extensions; python_version < "3.9"
 coverage[toml] >=7.6.1
 mypy ==1.14.1
 ruff ==0.14.1

tests/test_main.py

@@ -1,6 +1,10 @@
 import pickle
 
-from typing_extensions import Annotated, get_type_hints
+try:
+    from typing import Annotated, get_type_hints
+except ImportError:
+    # Python < 3.9
+    from typing_extensions import Annotated, get_type_hints
 
 from annotated_doc import Doc