install outcome v2 in tox's check stage

Author: graingert

tox.ini

@@ -102,6 +102,7 @@ description = "Run type checks: mypy on all platforms, and pyright on `src/trio[
 deps =
     -r test-requirements.txt
     exceptiongroup
+    https://github.com/graingert/outcome/archive/refs/heads/invalid-objects-on-unwrap.zip
 base_python = 3.10
 set_env =
     PYRIGHT_PYTHON_IGNORE_WARNINGS=1