time.process_time() instead of time.clock()

[isidroas]

Documentation removed in python/cpython@b6a09ae

This solves this error in my Python3.9

$ python sudoku.py
All tests pass.
Traceback (most recent call last):
  File "/tmp/pytudes/py/sudoku.py", line 158, in <module>
    solve_all(open("sudoku-easy50.txt"), "easy")
  File "/tmp/pytudes/py/sudoku.py", line 134, in solve_all
    times, results = zip(*[time_solve(grid) for grid in grids])
  File "/tmp/pytudes/py/sudoku.py", line 134, in <listcomp>
    times, results = zip(*[time_solve(grid) for grid in grids])
  File "/tmp/pytudes/py/sudoku.py", line 141, in time_solve
    start = time.clock()
AttributeError: module 'time' has no attribute 'clock'