stplib: Adopt to Python3

In Python3, cmp() method is no longer supported and numerical
operations evaluates value type more strictly.
So, stplib get some errors in its calculating process.

This patch fixes these problems and enable to use stplib on
Python3 interpreter.

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/lib/packet/bpdu.py

@@ -379,7 +379,7 @@ class ConfigurationBPDUs(bpdu):
 
     @staticmethod
     def _encode_timer(timer):
-        return timer * 0x100
+        return int(timer) * 0x100
 
 
 @bpdu.register_bpdu_type

ryu/lib/stplib.py

@@ -169,6 +169,13 @@ class EventPacketIn(event.EventBase):
         self.msg = msg
 
 
+# For Python3 compatibility
+# Note: The following is the official workaround for cmp() in Python2.
+# https://docs.python.org/3.0/whatsnew/3.0.html#ordering-comparisons
+def cmp(a, b):
+    return (a > b) - (a < b)
+
+
 class Stp(app_manager.RyuApp):
     """ STP(spanning tree) library. """
 
@@ -351,7 +358,7 @@ class Stp(app_manager.RyuApp):
 
     @staticmethod
     def _cmp_value(value1, value2):
-        result = cmp(value1, value2)
+        result = cmp(str(value1), str(value2))
         if result < 0:
             return SUPERIOR
         elif result == 0: