Fix cases of inconsistent namespacing in tests
Sometimes e.g. Series and pd.Series are used
in the same test file. This fixes some of these
cases, generally by using the explicitly
imported class.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

so ideally we would add these to the pre-commit rules that checks this. these are likley inconsistent in more places.

so this PR is ok, but would prefer that you change them across the entire codebase so that we are enforcing the rule. (and it may make sense to not change everything, e.g. do just Grouper, or just the Index ones). I am not sure how big a change this is. For context, this follows on from @dsaxton 's work on making the namespace using more consistent. Will modify the `scripts/check_for_inconsistent_pandas_namespace.py` file accordingly once this and the rest of the fixes of these kind are in  > I am not sure how big a change this is.

It's not too big, should get them done today along with the expanded CI check (see https://github.com/MarcoGorelli/PyDataGlobal2020-sprint/issues/1 ) > > I am not sure how big a change this is.
> 
> It's not too big, should get them done today along with the expanded CI check (see [MarcoGorelli/PyDataGlobal2020-sprint#1](https://github.com/MarcoGorelli/PyDataGlobal2020-sprint/issues/1) )

ok sounds great. feel free to merge these on green. Thanks @lpkirwin 