Run pyupgrade on core/groupby (#6351)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

xarray/core/groupby.py

@@ -1,6 +1,8 @@
+from __future__ import annotations
+
 import datetime
 import warnings
-from typing import Any, Callable, Hashable, Sequence, Union
+from typing import Any, Callable, Hashable, Sequence
 
 import numpy as np
 import pandas as pd
@@ -869,9 +871,9 @@ def _combine(self, applied, shortcut=False):
     def reduce(
         self,
         func: Callable[..., Any],
-        dim: Union[None, Hashable, Sequence[Hashable]] = None,
+        dim: None | Hashable | Sequence[Hashable] = None,
         *,
-        axis: Union[None, int, Sequence[int]] = None,
+        axis: None | int | Sequence[int] = None,
         keep_attrs: bool = None,
         keepdims: bool = False,
         shortcut: bool = True,
@@ -997,9 +999,9 @@ def _combine(self, applied):
     def reduce(
         self,
         func: Callable[..., Any],
-        dim: Union[None, Hashable, Sequence[Hashable]] = None,
+        dim: None | Hashable | Sequence[Hashable] = None,
         *,
-        axis: Union[None, int, Sequence[int]] = None,
+        axis: None | int | Sequence[int] = None,
         keep_attrs: bool = None,
         keepdims: bool = False,
         **kwargs: Any,