Revert "Coverage bug hunt"

This reverts commit 0628d22.

Author: Pawel-Kranzberg

pandas/_libs/tslibs/dtypes.pyx

@@ -78,7 +78,6 @@ _period_code_map = {
     "Q-NOV": 2011,    # Quarterly - November year end
 
     "M": 3000,        # Monthly - month end
-    "MS": 3000,       # Monthly - beginning of month
 
     "W-SUN": 4000,    # Weekly - Sunday end of week
     "W-MON": 4001,    # Weekly - Monday end of week
@@ -111,7 +110,7 @@ _period_code_map.update({
     "A": 1000,   # Annual
     "W": 4000,   # Weekly
     "C": 5000,   # Custom Business Day
-    # "MS": 3000,  # Monthly - beginning of month
+    "MS": 3000,  # Monthly - beginning of month
 })
 
 cdef set _month_names = {