Bug 1667288 - [moznetwork] Ensure interface names returned by _get_interface_list are strings, even on python 3; r=bc

Currently on my Ubuntu laptop, when running under python 3, _get_interface_list() returns a list
of (byte, string), like (b'lo', '127.0.0.1') which is both inconsistent and troublesome for name
comparisons like
https://searchfox.org/mozilla-central/rev/89d33e1c3b0a57a9377b4815c2f4b58d933b7c32/testing/mozbase/moznetwork/moznetwork/moznetwork.py#145.
Let's return (string, string) instead, regardless of py2/3.

Differential Revision: https://phabricator.services.mozilla.com/D91383

testing/mozbase/moznetwork/moznetwork/moznetwork.py

@@ -53,7 +53,7 @@ def _get_interface_list():
             namestr = names.tobytes()
         else:
             namestr = names.tostring()
-        return [(namestr[i:i + 32].split(b'\0', 1)[0],
+        return [(six.ensure_str(namestr[i:i + 32].split(b'\0', 1)[0]),
                  socket.inet_ntoa(namestr[i + 20:i + 24]))
                 for i in range(0, outbytes, struct_size)]