Create a centralised pytest fixture to provide controllable stdin and
captured stdout in all tests. Simplify DummyIO/DummyIn and remove the
custom DummyOut implementation and make use of pytest builtin fixtures.
Create a centralised pytest fixture to provide controllable stdin and
captured stdout that can be applied to any tests, regardless whether
they are based on pytest or unittest.
* `io` fixture can be used as a fixture in pytest-based tests
* `IOMixin` can be used to attach `io` attribute to any test class,
including `unittest.TestCase`
