Make pandas optional in workday calendar example (#30660)

The workday calendar expected pandas to be available and it is part
of our examples, however Airflow does not have pandas as a core
dependency, so in case someone does not have pandas installed, importing
of the workday example would fail.

This change makes pandas optional and fallbacks to regular working
days for the example in case it is not available (including warning
about it). It also fixes a slight inefficiency where the
USFederalHoliday calendar has been created every time next workday
was calculated.

airflow/example_dags/plugins/workday.py

@@ -18,28 +18,38 @@
 """Plugin to demonstrate timetable registration and accommodate example DAGs."""
 from __future__ import annotations
 
+import logging
 from datetime import timedelta
 
 # [START howto_timetable]
-from pandas.tseries.holiday import USFederalHolidayCalendar
 from pendulum import UTC, Date, DateTime, Time
 
 from airflow.plugins_manager import AirflowPlugin
 from airflow.timetables.base import DagRunInfo, DataInterval, TimeRestriction, Timetable
 
+log = logging.getLogger(__name__)
+holiday_calendar = None
+
+try:
+    from pandas.tseries.holiday import USFederalHolidayCalendar
+
+    holiday_calendar = USFederalHolidayCalendar()
+except ImportError:
+    log.warning("Could not import pandas. Holidays will not be considered.")
+
 
 class AfterWorkdayTimetable(Timetable):
     def get_next_workday(self, d: DateTime, incr=1) -> DateTime:
-        cal = USFederalHolidayCalendar()
         next_start = d
         while True:
             if next_start.weekday() in (5, 6):  # If next start is in the weekend go to next day
                 next_start = next_start + incr * timedelta(days=1)
                 continue
-            holidays = cal.holidays(start=next_start, end=next_start).to_pydatetime()
-            if next_start in holidays:  # If next start is a holiday go to next day
-                next_start = next_start + incr * timedelta(days=1)
-                continue
+            if holiday_calendar is not None:
+                holidays = holiday_calendar.holidays(start=next_start, end=next_start).to_pydatetime()
+                if next_start in holidays:  # If next start is a holiday go to next day
+                    next_start = next_start + incr * timedelta(days=1)
+                    continue
             break
         return next_start