return NDFrame instead of NDFrameT to fix type-var error

Author: anilbey

pandas/core/resample.py

@@ -205,9 +205,7 @@ def __getattr__(self, attr: str):
 
     # error: Signature of "obj" incompatible with supertype "BaseGroupBy"
     @property
-    def obj(self) -> NDFrameT:  # type: ignore[override, type-var]
-        # error: Incompatible return value type (got "Optional[Any]",
-        # expected "NDFrameT")
+    def obj(self) -> NDFrame:
         return self.groupby.obj  # type: ignore[return-value]
 
     @property