input: Write a function to classify a triangle based on its side lengths.
  assert classify_triangle(8, 8, 8) == "Equilateral triangle"
  assert classify_triangle(8, 8, 12) == "Isosceles triangle"
  assert classify_triangle(8, 10, 12) == "Scalene triangle"
output: def classify_triangle(a, b, c):
      if a == b == c:
          return "Equilateral triangle"
      elif a == b or b == c or a == c:
          return "Isosceles triangle"
      else:
          return "Scalene triangle"