TYP: type up parts of series.py
More typing.
Hello @topper-123! 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 2020-01-12 13:32:45 UTC I think this is the best I can do here: I agree a TypeVar would be best, but it doesn't play well with ``._constructor`` AFAICS.

I wonder if we should deprecate ``._constructor`` and use ``type(self)`` for instantiation, but that's probably a different discussion wit its own traps. lgtm. ping Thanks @topper-123 