[4.6] capsys: ensure fd is unbuffered

[blueyed]

Fixes #5134.

[nicoddemus]

Tests are failing, regardless, @antocuni any chance of testing this branch for your case and see if it fixes it?

[blueyed]

Note that _py36_windowsconsoleio_workaround does something similar already:

pytest/src/_pytest/capture.py

Lines 804 to 816 in f7ecd72

	def _reopen_stdio(f, mode):
	if not buffered and mode[0] == "w":
	buffering = 0
	else:
	buffering = -1
	return io.TextIOWrapper(
	open(os.dup(f.fileno()), mode, buffering),
	f.encoding,
	f.errors,
	f.newlines,
	f.line_buffering,
	)

[blueyed]

I do not really feel like we should have it, since it is not required for py3 anyway, and might break other things.

[blueyed]

But FWIW tests are passing by now.

[blueyed]

I do not expect any feedback from @antocuni here (chatted with regard to the workaround he added to pyrepl). Might re-open when I look into that (pypy/pyrepl#22, pypy/pyrepl@3ff3cfa).