fix(subprocess): make sure empty value for PY3 and text mode is correct

[william-gr]

Without this patch, if a subprocess is in text mode and there is no stderr or stdout output it will default to empty bytes (b"") in python 3, instead of empty string ("").

[jamadden]

It seems to me that there is a reason for this behavior. Can you provide a test that passes on unpatched Python 2 and 3, but fails with a patched 2 and 3?

[coveralls]

Changes Unknown when pulling 0cb896a on william-gr:subprocess-py3k-fix into ** on gevent:master**.

[william-gr]

Hm, not sure what you're asking...

Take this for instance:

proc = subprocess.PIPE("ls 1>&2", shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE, encoding='utf-8')
proc.communicate()[0].split('\n')

Without this patch it will fail on python 3, because stdout will be b'', even though text mode was specified (encoding='utf8'). With this patch it will work because it will return "", as expected.
This is a no-op for python 2.

[william-gr]

@jamadden Oh, one more thing I forgot. This mimics the real python 3 subprocess behavior for empty stdout/stderr. Not sure if my fix is the better one since I am not sharp on gevent internals, but thats what I would expect as an end result.