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

I am not 100% sure this is something we want, or at least with this implementation.

cc @simonjayhawkins 
Hello @MomIsBestFriend! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-01-30 15:52:49 UTC > I am not 100% sure this is something we want, or at least with this implementation.

eventually.

typing was original done like this, but changed in #28134, since `_pytest.mark.structures.MarkDecorator` currently resolves to `Any`.

for the current status of typing in pytest see https://github.com/pytest-dev/pytest/issues/3342 I see that is related to https://github.com/pandas-dev/pandas/issues/28133, I thought about helping there, but I don't know how to do it without stub files.

So I am closing this :)

sorry for your time @simonjayhawkins 
