Make IO and TextIO properties abstract

[JukkaL]

Currently several properties are concrete, even though IO and TextIO are ABCs.

Originally python/mypy#266.

[JelleZijlstra]

If we do this, we should be careful to not accidentally make subclasses non-instantiable (#1476).

[AlexWaygood]

Note that #8726 tried to fix this, but the mypy_primer output indicated that it would have been hugely disruptive