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.
diff --git a/Lib/turtledemo/bytedesign.py b/Lib/turtledemo/bytedesign.py
index b3b095b..1b7452b 100755
--- a/Lib/turtledemo/bytedesign.py
+++ b/Lib/turtledemo/bytedesign.py
@@ -23,7 +23,7 @@
 """
 
 from turtle import Turtle, mainloop
-from time import clock
+from time import perf_counter as clock
 
 # wrapper for any additional drawing routines
 # that need to know about each other