BUG: inferred_freq results in None for eastern hemispheres timezones

[sinhrks]

Found during #7299.

dates = ['2000-01-31', '2000-02-29', '2000-03-31', '2000-04-30']
pd.DatetimeIndex(dates).inferred_freq
# M

pd.DatetimeIndex(dates, tz='US/Eastern').inferred_freq
# M

pd.DatetimeIndex(dates, tz='Mexico/General').inferred_freq
# M

pd.DatetimeIndex(dates, tz='Asia/Tokyo').inferred_freq
# None

pd.DatetimeIndex(dates, tz='Europe/Paris').inferred_freq
# None

pd.DatetimeIndex(dates, tz='Australia/Sydney').inferred_freq
# None

[jreback]

this should use localize the timestamps before returning i8: https://github.com/pydata/pandas/blob/master/pandas/tseries/frequencies.py#L649

[jreback]

fixes (I think just #7317) reverted, so this needs to be addressed again

@sinhrks

[jreback]

sorry...this is ok, its the Datetime.insert issue