python312Packages.pytest-trio: drop dependency on async-generator

The `async_generator` module is a backport of async generators to
Python 3.5 and it started failing on 3.12, but pulling it is safe.

pkgs/development/python-modules/pytest-trio/default.nix

···
 { lib, buildPythonPackage, fetchFromGitHub, pythonOlder
-, trio, async-generator, hypothesis, outcome, pytest }:
+, trio, hypothesis, outcome, pytest }:
 
 buildPythonPackage rec {
   pname = "pytest-trio";
···
 
   propagatedBuildInputs = [
     trio
-    async-generator
     outcome
   ];