REF: Create test_encoding file for CSV
This is 99.99% copy and paste
On board with the concept. What's the bit that isn't copy / paste? > What's the bit that isn't copy / paste?

* `BytesIO` isn't used in `test_common.py` anymore
* The module documentation in `test_encoding.py` is new

The 99.99% was a very pedantic figure so to speak. 🙂  thanks @gfyoung 