refactor GroupBy internals (#9389)

* More tests

* Refactoring GroupBy

1. Simplify ResolvedGrouper by moving logic to EncodedGroups
2. Stack outside ResolvedGrouper in GroupBy.__init__ to prepare for
   multi-variable GroupBy

* bail on pandas 2.0

Author: dcherian

xarray/core/coordinates.py

@@ -352,7 +352,7 @@ def _construct_direct(
         return obj
 
     @classmethod
-    def from_pandas_multiindex(cls, midx: pd.MultiIndex, dim: str) -> Self:
+    def from_pandas_multiindex(cls, midx: pd.MultiIndex, dim: Hashable) -> Self:
         """Wrap a pandas multi-index as Xarray coordinates (dimension + levels).
 
         The returned coordinates can be directly assigned to a