Update reference_resolver.py to use objects from Python typing library

This is failing with python 3.7 (Colab) there are a few tutorials that install tensorflow_docs for the video embedding.

PiperOrigin-RevId: 472816339

Author: tensorflower-gardener

tools/tensorflow_docs/api_generator/reference_resolver.py

@@ -14,6 +14,8 @@
 # ==============================================================================
 """Turn Python docstrings into Markdown for TensorFlow documentation."""
 
+from __future__ import annotations
+
 import collections
 import contextlib
 import html