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.