input: Write a python function to estimate the execution time of a given function.
  assert estimate_time(lambda: time.sleep(1)) >= 1
  assert estimate_time(lambda: sum(range(10000))) < 1
  assert estimate_time(lambda: [x**2 for x in range(1000)]) < 1
output: import time
  
  def estimate_time(func):
      start_time = time.time()
      func()
      end_time = time.time()
      return end_time - start_time