BUG: inferred_freq results in None for eastern hemispheres timezones
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
```
