capture: don't assume that the tmpfile is backed by a BytesIO
capture: don't assume that the tmpfile is backed by a BytesIO

Since tmpfile is a parameter to SysCapture, it shouldn't assume things
unnecessarily, when there is an alternative.

pytest-dev/pytest,The-Compiler/pytest,Akasurde/pytest,markshao/pytest,nicoddemus/pytest,nicoddemus/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest