DOC: make it possible to run doctests
This adds a small entry in the conftest.py file, which makes it possible to run doctests with eg

```
pytest --doctests-module pandas/core/series.py
```

We are far from being able to run them all without error, and if we would want that is even another question (it may be too much small details to pay attention to), but this let's you at least run specific doctests.
@jorisvandenbossche this is great. I would propose you add a script ``ci/script_doctests.py``, add it in ``.travis.yml`` after ``script_multi.py`` and you just put the actual testing command / commands as we add things.

So then as things are fixed can be added there w/o impacting anything else. if you want to get really fancy we can add an env variable DOC_TESTS to only run them on selective builds (e.g. 3.6/2.7). > We are far from being able to run them all without error, and if we would want that is even another question (it may be too much small details to pay attention to), but this let's you at least run specific doctests.

If we can automate the running, why not. Having manually run tests is like not having tests :> > If we can automate the running, why not.

Because we may find it is too much work to keep it working. The doctesting ensures the example are correct (which is important!), but it is also quite nitpicky and it has to be exactly correct. 
https://github.com/pandas-dev/pandas/pull/15625 is an example of adjustments needed to make them pass (but that PR also fixed an actual error in the example, apart from more stylistic changes). So might be good to first get some experience with to see how it goes.
 ok. well if they are fixed in a particular file we should then test them to *keep* them working :> Yes, we can certainly add that when certain files pass