fix typing

Author: jorisvandenbossche

pandas/_testing/asserters.py

@@ -808,11 +808,11 @@ def assert_extension_array_equal(
     # Specifically for StringArrayNumpySemantics, validate here we have a valid array
     if isinstance(left.dtype, StringDtype) and left.dtype.storage == "python_numpy":
         assert np.all(
-            [np.isnan(val) for val in left._ndarray[left_na]]
+            [np.isnan(val) for val in left._ndarray[left_na]]  # type: ignore[attr-defined]
         ), "wrong missing value sentinels"
     if isinstance(right.dtype, StringDtype) and right.dtype.storage == "python_numpy":
         assert np.all(
-            [np.isnan(val) for val in right._ndarray[right_na]]
+            [np.isnan(val) for val in right._ndarray[right_na]]  # type: ignore[attr-defined]
         ), "wrong missing value sentinels"
 
     left_valid = left[~left_na].to_numpy(dtype=object)

pandas/core/arrays/string_.py

@@ -718,7 +718,7 @@ def _from_sequence(
             dtype = StringDtype(storage="python_numpy")
         return super()._from_sequence(scalars, dtype=dtype, copy=copy)
 
-    def _from_backing_data(self, arr: np.ndarray) -> NumpyExtensionArray:
+    def _from_backing_data(self, arr: np.ndarray) -> StringArrayNumpySemantics:
         # need to overrde NumpyExtensionArray._from_backing_data to ensure
         # we always preserve the dtype
         return NDArrayBacked._from_backing_data(self, arr)