test_sodapro.py: use ME instead of M

Author: kandersolar

tests/iotools/test_sodapro.py

@@ -21,7 +21,7 @@
 
 index_verbose = pd.date_range('2020-06-01 12', periods=4, freq='1min',
                               tz='UTC')
-index_monthly = pd.date_range('2020-01-01', periods=4, freq='1M')
+index_monthly = pd.date_range('2020-01-01', periods=4, freq='1ME')
 
 
 dtypes_mcclear_verbose = [