Make kind argument in CFTimeIndex._maybe_cast_slice_bound optional (#5359)

* [test-upstream] Make kind argument in CFTimeIndex._maybe_cast_slice_bound optional

* Update doc/whats-new.rst

Co-authored-by: keewis <keewis@users.noreply.github.com>

Co-authored-by: keewis <keewis@users.noreply.github.com>

Author: spencerkclark

doc/whats-new.rst

@@ -33,6 +33,10 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+- Fix a minor incompatibility between partial datetime string indexing with a
+  :py:class:`CFTimeIndex` and upcoming pandas version 1.3.0 (:issue:`5356`,
+  :pull:`5359`).
+  By `Spencer Clark <https://github.com/spencerkclark>`_.
 
 
 Documentation

xarray/coding/cftimeindex.py

@@ -465,9 +465,14 @@ def get_loc(self, key, method=None, tolerance=None):
         else:
             return pd.Index.get_loc(self, key, method=method, tolerance=tolerance)
 
-    def _maybe_cast_slice_bound(self, label, side, kind):
+    def _maybe_cast_slice_bound(self, label, side, kind=None):
         """Adapted from
-        pandas.tseries.index.DatetimeIndex._maybe_cast_slice_bound"""
+        pandas.tseries.index.DatetimeIndex._maybe_cast_slice_bound
+
+        Note that we have never used the kind argument in CFTimeIndex and it is
+        deprecated as of pandas version 1.3.0.  It exists only for compatibility
+        reasons.  We can remove it when our minimum version of pandas is 1.3.0.
+        """
         if not isinstance(label, str):
             return label