BUG: Concat typing (pandas-dev#36409)

Author: rhshadrach

pandas/core/reshape/concat.py

@@ -31,6 +31,7 @@
 
 if TYPE_CHECKING:
     from pandas import DataFrame
+    from pandas.core.generic import NDFrame
 
 # ---------------------------------------------------------------------
 # Concatenate DataFrame objects
@@ -54,7 +55,7 @@ def concat(
 
 @overload
 def concat(
-    objs: Union[Iterable[FrameOrSeries], Mapping[Label, FrameOrSeries]],
+    objs: Union[Iterable["NDFrame"], Mapping[Label, "NDFrame"]],
     axis=0,
     join: str = "outer",
     ignore_index: bool = False,
@@ -69,7 +70,7 @@ def concat(
 
 
 def concat(
-    objs: Union[Iterable[FrameOrSeries], Mapping[Label, FrameOrSeries]],
+    objs: Union[Iterable["NDFrame"], Mapping[Label, "NDFrame"]],
     axis=0,
     join="outer",
     ignore_index: bool = False,