Added __getattr__ in DataFrameGroupBy (#457)

* ty

* changed the return value to SeriesGroupBy and corrected the tests likewise

* changed test to test '__getattr()' indirectly

* modified the tests so SeriesGroupBy is not tested directly but indirectly

Author: ramvikrams

pandas-stubs/core/groupby/generic.pyi

@@ -359,3 +359,4 @@ class DataFrameGroupBy(GroupBy):
         ascending: bool = ...,
         dropna: bool = ...,
     ) -> Series[float]: ...
+    def __getattr__(self, name: str) -> SeriesGroupBy: ...

tests/test_frame.py

@@ -2292,3 +2292,14 @@ def test_groupby_and_transform() -> None:
     check(assert_type(c2, pd.DataFrame), pd.DataFrame)
     check(assert_type(c3, pd.Series), pd.Series)
     check(assert_type(c4, pd.Series), pd.Series)
+
+
+def test_getattr_and_dataframe_groupby() -> None:
+    df = pd.DataFrame(
+        data={"col1": [1, 1, 2], "col2": [3, 4, 5], "col3": [0, 1, 0], 0: [-1, -1, -1]}
+    )
+    check(assert_type(df.groupby("col1").col3.agg(min), pd.Series), pd.Series)
+    check(
+        assert_type(df.groupby("col1").col3.agg([min, max]), pd.DataFrame),
+        pd.DataFrame,
+    )