Add interchange to pandas-stubs/api/__init__.pyi (#713)

* Add interchange to pandas-stubs/api/__init__.pyi

* Add test

Author: luke396

pandas-stubs/api/__init__.pyi

@@ -1,5 +1,6 @@
 from pandas.api import (
     extensions as extensions,
     indexers as indexers,
+    interchange as interchange,
     types as types,
 )

tests/test_api_types.py

@@ -470,3 +470,11 @@ def check_ext_dtype(etype: Type[ExtensionDtype]):
     check_ext_dtype(pd.SparseDtype)
     check_ext_dtype(pd.Float32Dtype)
     check_ext_dtype(pd.Float64Dtype)
+
+
+def test_from_dataframe() -> None:
+    # GH 712
+    check(
+        assert_type(pd.api.interchange.from_dataframe(dframe), pd.DataFrame),
+        pd.DataFrame,
+    )