Replace time.clock() with time.perf_counter() in slowsquare examples

Fixes #1757

time.clock() was removed in Python 3.8 in favor of time.perf_counter().
This commit updates the slowsquare RPC examples to use the modern API.

Changes:
- examples/twisted/wamp/rpc/slowsquare/frontend.py: 3 replacements
- examples/asyncio/wamp/rpc/slowsquare/frontend.py: 3 replacements

time.perf_counter() provides a monotonic clock with the best available
resolution for measuring performance, which is the intended behavior
for timing RPC call durations.

Tested with: just test-integration-ab-examples from crossbar repository

Author: oberstet

examples/asyncio/wamp/rpc/slowsquare/frontend.py

@@ -40,14 +40,14 @@ class Component(ApplicationSession):
     async def onJoin(self, details):
         def got(started, msg, f):
             res = f.result()
-            duration = 1000.0 * (time.clock() - started)
+            duration = 1000.0 * (time.perf_counter() - started)
             print("{}: {} in {}".format(msg, res, duration))
 
-        t1 = time.clock()
+        t1 = time.perf_counter()
         d1 = self.call("com.math.slowsquare", 3, 2)
         d1.add_done_callback(partial(got, t1, "Slow Square"))
 
-        t2 = time.clock()
+        t2 = time.perf_counter()
         d2 = self.call("com.math.square", 3)
         d2.add_done_callback(partial(got, t2, "Quick Square"))
 

examples/twisted/wamp/rpc/slowsquare/frontend.py

@@ -41,14 +41,14 @@ def onJoin(self, details):
         print("session attached")
 
         def got(res, started, msg):
-            duration = 1000.0 * (time.clock() - started)
+            duration = 1000.0 * (time.perf_counter() - started)
             print("{}: {} in {}".format(msg, res, duration))
 
-        t1 = time.clock()
+        t1 = time.perf_counter()
         d1 = self.call("com.math.slowsquare", 3)
         d1.addCallback(got, t1, "Slow Square")
 
-        t2 = time.clock()
+        t2 = time.perf_counter()
         d2 = self.call("com.math.square", 3)
         d2.addCallback(got, t2, "Quick Square")