REF: implement ExtensionIndex._concat_same_dtype, use for IntervalIndex
Hello @jbrockmendel! 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 2020-02-08 02:13:36 UTC cc @jorisvandenbossche  thanks @jbrockmendel 