Make Debugger.interaction match the signature of Pdb.interaction.

This allows pudb to be used as debugger for pytest>=3.0, e.g.

```
$ cat test_foo.py
def test_1():
    assert False
$ pytest --pdbcls pudb.debugger:Debugger --pdb --capture=no
```
(starts pudb at the failed assertion site).

Note the need to pass `--capture=no` (or its synonym `-s`) as otherwise
pytest tries to manage the standard streams itself.

Backwards compatibility is preserved.