Apply ruff/flake8-pyi rule PYI034

PYI034 such methods usually return `self` at runtime

Author: DimitriPapadopoulos

pyproject.toml

@@ -284,6 +284,7 @@ ignore = [
 # don't enforce absolute imports
 "asv_bench/**" = ["TID252"]
 # looks like ruff bugs
+"xarray/core/_typed_ops.py" = ["PYI034"]
 "xarray/namedarray/_typing.py" = ["PYI018"]
 
 [tool.ruff.lint.isort]

xarray/core/accessor_dt.py

@@ -5,6 +5,7 @@
 
 import numpy as np
 import pandas as pd
+from typing_extensions import Self
 
 from xarray.coding.calendar_ops import _decimal_year
 from xarray.coding.times import infer_calendar_name
@@ -650,7 +651,7 @@ def total_seconds(self) -> T_DataArray:
 class CombinedDatetimelikeAccessor(
     DatetimeAccessor[T_DataArray], TimedeltaAccessor[T_DataArray]
 ):
-    def __new__(cls, obj: T_DataArray) -> CombinedDatetimelikeAccessor:
+    def __new__(cls, obj: T_DataArray) -> Self:
         # CombinedDatetimelikeAccessor isn't really instantiated. Instead
         # we need to choose which parent (datetime or timedelta) is
         # appropriate. Since we're checking the dtypes anyway, we'll just