input: Write a python function to measure the execution time of a given function using a decorator.
  assert timeit(lambda: sum(range(10000)))() == sum(range(10000))
  assert timeit(lambda x: x**2)(10) == 100
  assert timeit(lambda s: s[::-1])("hello") == "olleh"
output: import time
  
  def timeit(func):
      def wrapper(*args, **kwargs):
          start_time = time.time()
          result = func(*args, **kwargs)
          end_time = time.time()
          print(f"Execution time: {end_time - start_time:.6f} seconds")
          return result
      return wrapper