> Is this still the state of the art?  What about parsing overhead?

The io module has three modules:
- io.py just imports everything from _io
- _io is the default C implementation
- _pyio.py must be imported explicitly to get the pure Python implementation

=> no parsing overhead for the default case of importing the C implementation

> Is there direct regrtest support for this?

You can take a look at test_io, test_memoryio or test_heapq for inspiration.