tests: assume that `raw` attribute is present on original socket file object It seems like the original socket file object is always an io.BufferedIO instance. If not, the code will fail and we should try harder to get the socket object (e.g. if the original socket file object is unbuffered, we can get the `_sock` attribute directly from it).

tests: constant-fold a `pycompat.ispy3` in testlib/badserverext.py

tests: remove Python 2 special cases in test-stdio.py