[SymForce] Add typing_extensions for py38

This is a required install dependency.  There are other things that also
depend on it transitively in the dev requirements; it might be nice to
test that SymForce is importable with just the raw install requirements?

Topic: sf-typing-extensions-py38

Author: aaron-skydio

requirements_build.txt

@@ -28,5 +28,7 @@ file:./gen/python
     # via symforce (pyproject.toml)
 sympy
     # via symforce (pyproject.toml)
+typing-extensions
+    # via symforce (pyproject.toml)
 wheel
     # via symforce (pyproject.toml)

requirements_dev_py38.txt

@@ -333,6 +333,7 @@ types-setuptools==75.8.0.20250110
     # via symforce (pyproject.toml)
 typing-extensions==4.12.2
     # via
+    #   symforce (pyproject.toml)
     #   beautifulsoup4
     #   ipython
     #   mistune

setup.py

@@ -390,6 +390,7 @@ def fixed_readme() -> str:
                 f"skymarshal @ file://localhost/{ESCAPED_SOURCE_DIR}/third_party/skymarshal",
                 "sympy~=1.11.1",
                 f"symforce-sym @ file://localhost/{ESCAPED_SOURCE_DIR}/gen/python",
+                "typing-extensions; python_version<'3.9'",
             ]
         ),
         setup_requires=setup_requirements,