[_io] Turn _IOBase.read() and write() into functions - #16583

Originally introduced in #4912, because the docs say:

Even though IOBase does not declare read() or write() because their signatures will vary, implementations and clients should consider those methods part of the interface.

In the PR I said:

The change seems correct to me and the best solution. I am still a bit wary that this might cause unforeseen problems, but the mypy_primer output makes me optimistic. Let's try this an be prepared to revert if it causes problems.

This now seems to be the case, since ty 0.0.87 not very unreasonably complains in sub-classes (see #16551):

write: Override does not accept writes of type (...) -> Any

Since the interface may vary according to the docs, I'm not sure how useful there fields are in any case. Let's first see what primer says.