monthly periods prior to dec-1969 off by one year

[mattknoxca]

# version 0.8.0 of pandas
# version 1.6.1 of numpy
# windows XP 32-bit

>>> import pandas as pd
>>> pd.Period('1969-12', 'M')
Period('Dec-1969', 'M')
>>> pd.Period('1969-11', 'M')
Period('Nov-1970', 'M')