BUG: Inconsistent datetime comparison with Tz 
Related to #8306. On current master, `Timestamp` comparison results in `TypeError` if its timezones are different. However, `Index` and `Series` implicitly converts tz to GMT

```
pd.Timestamp('2016-01-01 12:00', tz='US/Eastern') > pd.Timestamp('2016-01-01 08:00')
# TypeError: Cannot compare tz-naive and tz-aware timestamps

# same result as idx.tz_convert(None) > pd.Timestamp('2016-01-01 08:00')
idx = pd.date_range('2016-01-01 12:00', periods=10, freq='H', tz='Asia/Tokyo')
idx > pd.Timestamp('2016-01-01 08:00')
# array([False, False, False, False, False, False,  True,  True,  True,  True], dtype=bool)
```

Numeric ops raises `TypeError` as expected.

```
idx - pd.Timestamp('2016-01-01 08:00')
# TypeError: Timestamp subtraction must have the same timezones or no timezones
```
