Separate creation of inputreader

I promise this one is the last O:-).

Currently, those importers that extended `__init__` opened files two times. And couple of non-used instance variables were created as well. This is my take on it. However, I am not sure, whether separating just something that would return just encoding name and still creating `inputreader` in `__init__` wouldn't be even better, because the code is the same up to encoding name :-/. What do you think?