Fix tests for UTC >=+ 4 timezones
Thanks!
Currently test_timestamp_field_from_ts assumes the timezone to be less than +4 so it remains the same day. This would fail when running with for example UTC+8:

```
======================================================================
FAIL: test_timestamp_field_parts (tests.fields.TestTimestampField)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/build/python-peewee/src/peewee-3.13.1/tests/fields.py", line 1066, in test_timestamp_field_parts
    self.assertEqual(d, 2)
AssertionError: 1 != 2

----------------------------------------------------------------------
Ran 859 tests in 3.450s

FAILED (failures=1, skipped=102)
```

Just comparing it with dt_utc.day fixes the failure.