Move `BufferedIOBase.raw` to sub-classes

[srittau]

[srittau]

Let's judge the impact of removing this.

[github-actions[bot]]

Diff from mypy_primer, showing the effect of this PR on open source code:

mypy_primer (https://github.com/hauntsaninja/mypy_primer)
+ mypy_primer/utils.py:172: error: "BufferedReader" has no attribute "raw"  [attr-defined]
+ mypy_primer/utils.py:173: error: Unused "type: ignore" comment  [unused-ignore]

[github-actions[bot]]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Originally added in #3483, in which the author was already wondering whether to put it on the sub-classes. But the lazy reviewer didn't look closer.