input: Write a function to generate a crossword puzzle with random letters.
  assert len(generate_crossword()) == 10
  assert len(generate_crossword()[0]) == 10
  assert all(char.isalpha() for row in generate_crossword() for char in row)
output: import random
  import string
  
  def generate_crossword():
      size = 10
      crossword = []
      for _ in range(size):
          row = ''.join(random.choice(string.ascii_uppercase) for _ in range(size))
          crossword.append(row)
      return crossword