STY/TST: what standard to follow to import top-level objects in tests
So this discussion regularly comes up (latest one being https://github.com/pandas-dev/pandas/pull/33184), so let's try to decide which one of the two we prefer.

In test code, do we do:

```
from pandas import Series
...
Series(...)
```

or 

```
import pandas as pd
...
pd.Series(..)
```

Personally, I prefer the second, and from recent discussions I have the impression @WillAyd and @datapythonista do as well (is that correct?)

To be clear: consistency within a single file is most important (so not mixing Series and pd.Series in a single file). 
But *when* someone does a PR to fix such inconsistencies in a file, it would be good if there is agreement on whether such a PR should replace `pd.Series` with `Series` or rather replace `Series` with `pd.Series`.
