Previously, "import _io" worked on both CPython and MicroPython (essentially
by a chance on CPython, as there's not guarantee that its contents will stay
the same across versions), but as the module was renamed to uio, need to use
more robust import sequence for compatibility.
Functionality we provide in builtin io module is fairly minimal. Some
code, including CPython stdlib, depends on more functionality. So, there's
a choice to either implement it in C, or move it _io, and let implement other
functionality in Python. 2nd choice is pursued. This setup matches CPython
too (_io is builtin, io is Python-level).
