docs/requirements: add typing_extensions

This fixes failing builds on readthedocs.org.

    WARNING: autodoc: failed to import module 'firmware' from module 'pybricksdev'; the following exception was raised:
    No module named 'typing_extensions'

Author: dlech

docs/requirements.txt

@@ -3,3 +3,4 @@
 Sphinx==4.0.2
 sphinx-rtd-theme==0.5.2
 toml==0.10.2
+typing_extensions==4.3.0