`IO` is purported to be the type of the file objects returned by `open`.
However, all methods on those objects take positional-only arguments, while
`IO`'s methods are declared with regular arguments. As such, the file objects
cannot actually be considered to implement `IO`. The same thing applies to
`BinaryIO`.

Fix this by adjusting the definition of these ABCs to match the file objects.

This is technically a breaking change, but it is unlikely to actually break
anything:

* These methods should never be called at runtime, since they are abstract.
  Therefore, this should not cause any runtime errors.

* In typeshed these arguments are already positional-only, so this should
  not cause any errors during typechecking either.

Correct the description of the default state of test module compilation.