python3Packages.pytest-trio: add mit license to meta.license

https://github.com/python-trio/pytest-trio/blob/250ec45f5d72eca78ab0ea2d1571667d2c51f8b5/setup.py#L31-L32

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

···
   meta = {
     description = "Pytest plugin for trio";
     homepage = "https://github.com/python-trio/pytest-trio";
-    license = lib.licenses.asl20;
+    license = with lib.licenses; [
+      asl20
+      mit
+    ];
     maintainers = with lib.maintainers; [ hexa ];
   };
 }