Address mypy failure

Author: ssbarnea

src/tox/tox_env/python/virtual_env/package/util.py

@@ -4,7 +4,8 @@
 from typing import TYPE_CHECKING, Optional, Set, cast
 
 if TYPE_CHECKING:
-    from packaging.markers import Marker, Op, Variable
+    from packaging._parser import Op, Variable
+    from packaging.markers import Marker
     from packaging.requirements import Requirement