TST,CHANGELOG: add test and whatsnew entry for dirint scalar fix GH#2751

Author: Omesh37

docs/sphinx/source/whatsnew/v0.15.2.rst

@@ -14,6 +14,9 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+* Fixed :py:func:`pvlib.irradiance.dirint` raising ``KeyError`` with
+  scalar inputs on pandas >= 2.0. (:pull:`XXXX`, :issue:`2751`)
+  By :ghuser:`Omesh37`.
 * Corrects a bug in :py:func:`pvlib.temperature.fuentes`. If inputs were
   data type integer, users can expect modeled cell temperature values to
   increase slightly.