input: Write a python function to count the occurrence of each character in a string.
  assert count_characters('python') == {'p': 1, 'y': 1, 't': 1, 'h': 1, 'o': 1, 'n': 1}
  assert count_characters('hello') == {'h': 1, 'e': 1, 'l': 2, 'o': 1}
  assert count_characters('mississippi') == {'m': 1, 'i': 4, 's': 4, 'p': 2}
output: def count_characters(s):
      char_count = {}
      for char in s:
          if char in char_count:
              char_count[char] += 1
          else:
              char_count[char] = 1
      return char_count