add explanatory comment

Author: kandersolar

pvlib/iotools/meteonorm.py

@@ -532,6 +532,8 @@ def _get_meteonorm(
 
     # Check for None type in case of TMY request
     # Check for DateParseError in case of relative times, e.g., '+3hours'
+    # TODO: remove ValueError when our minimum pandas version is high enough
+    # to make it unnecessary (2.0?)
     if (start is not None) & (start != 'now'):
         try:
             start = pd.Timestamp(start)