input: Write a python function to estimate the value of π using the Gregory-Leibniz series.
  assert abs(estimate_pi(10) - 3.0418396189294032) < 0.01
  assert abs(estimate_pi(100) - 3.1315929035585537) < 0.01
  assert abs(estimate_pi(1000) - 3.140592653839794) < 0.01
output: def estimate_pi(n):
      pi_estimate = 0
      for k in range(n):
          pi_estimate += (-1)**k / (2*k + 1)
      return 4 * pi_estimate