fix nw.LazyFrame missing _version ClassVar

Author: camriddell

narwhals/dataframe.py

@@ -2361,6 +2361,8 @@ class LazyFrame(BaseFrame[LazyFrameT]):
         ```
     """
 
+    _version: ClassVar[Version] = Version.MAIN
+
     @property
     def _compliant(self) -> CompliantLazyFrame[Any, LazyFrameT, Self]:
         return self._compliant_frame