In python type-hinting, how can I make an argument accept any subclass of a base class?

If you annotate a function argument with the base class (io.IOBase in your case) then you can also pass instances of any subtype of the base class – inheritance applies to annotation types as well.

That said, you could use typing.IO as a generic type representing any I/O stream (and typing.TextIO and typing.BinaryIO for binary and text I/O streams respectively).