Python 3.9 code mixed with Python 3.10 code while Python 3.10 is not supported

[AnyCPU]

This commit 40aab6d
introduces Python 3.10 code in codebase mainly based on Python 3.9.

At the moment the Kats package can be installed only for Python 3.9 and in minimal mode MINIMAL_KATS=1.

Diff sample:

+from kats.compat.pandas import assert_frame_equal, assert_series_equal
+
FigSize = Tuple[int, int]
-INTERPOLATION_METHOD_TYPE = (
-    Literal["higher"]
-    | Literal["linear"]
-    | Literal["lower"]
-    | Literal["midpoint"]
-    | Literal["nearest"]
-)
+INTERPOLATION_METHOD_TYPE = Union[
+    Literal["higher"],
+    Literal["linear"],
+    Literal["lower"],
+    Literal["midpoint"],
+    Literal["nearest"],
+]