PERF: avoid object-dtype comparisons
Looking at the --durations=10 results in the tests, one that stands out is

```
7.08s call     pandas/tests/series/test_datetime_values.py::TestSeriesDatetimeValues::test_dt_namespace_accessor
```

It turns out all but about 0.15 seconds of this is spent in a `tm.assert_series_equal` call for a Datetime64TZ Series.  Tracking that down, its in array_equivalent, which casts to object-dtype before comparing.  This avoids that casting, shaves almost 7 seconds off that test.