time.clock() now emits a DeprecationWarning (GH-4020)

bpo-31803: time.clock() and time.get_clock_info('clock') now emit a
DeprecationWarning warning.

Replace time.clock() with time.perf_counter() in tests and demos.

Remove also hasattr(time, 'monotonic') in test_time since time.monotonic()
is now always available since Python 3.5.

Lib/turtledemo/tree.py

@@ -16,7 +16,7 @@ the current pen is cloned. So in the end
 there are 1024 turtles.
 """
 from turtle import Turtle, mainloop
-from time import clock
+from time import perf_counter as clock
 
 def tree(plist, l, a, f):
     """ plist is list of pens