Add missing dependency on typing-extensions

This was present at v4.6.0+ in our testsuite context via

    pytest -> exceptiongroup -> typing-extensions

Adding it to our explicit dependencies prevents failures for users on
3.9 and 3.10 .

A new test scenario cannot be trivially added for this, since any testing
would need to be done without pytest. Automated testing is therefore
considered out-of-scope for this change.

Manual tests show that `pip-compile -h` fails on 3.9 before this change
and pass afterwards.

Author: sirosen

changelog.d/2424.bugfix.md

@@ -0,0 +1,2 @@
+Added a missing dependency on ``typing-extensions`` for Python 3.9 and 3.10
+-- by {user}`sirosen`.

pyproject.toml

@@ -42,6 +42,7 @@ dependencies = [
   "click >= 8",
   "pip >= 22.2",
   "pyproject_hooks",
+  "typing_extensions >= 4.6.0; python_version < '3.11'",
   "tomli; python_version < '3.11'",
   # indirect dependencies
   "setuptools", # typically needed when pip-tools invokes setup.py