Support only RawIOBase in io_compat tests
Support only RawIOBase in io_compat tests

It didn't make any sense to keep compatibility with other `IOBase`
implementations since `RawIOChunk` inherits from `RawIOBase`.

Also `StringIO`, the only implementation that didn't inherit
`RawIOBase`, behaves slightly different because it's text-based,
something will never happend with `RawIOChunk`.

This simplified a little bit the io_compat tests.

Terseus/python-io-chunks