test: make probe_ports a class method under python2.

Author: rouilj

test/wsgi_liveserver.py

@@ -84,7 +84,8 @@ def _post_teardown(self):
             self._thread.join()
             del self._server
 
-    def probe_ports(start=port_range[0], end=port_range[1]):
+    @classmethod
+    def probe_ports(cls, start=port_range[0], end=port_range[1]):
         s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
         port = start