input: Write a function to count the frequency of each character in a given string.
  assert count_characters("A quick brown fox jumps over the lazy dog.") == {'A': 1, ' ': 8, 'q': 1, 'u': 2, 'i': 1, 'c': 1, 'k': 1, 'b': 1, 'r': 2, 'o': 4, 'w': 1, 'n': 1, 'f': 1, 'x': 1, 'j': 1, 'm': 1, 'p': 1, 's': 1, 'v': 1, 'e': 1, 't': 1, 'l': 1, 'a': 1, 'z': 1, 'y': 1, 'd': 1, 'g': 1, '.': 1}
  assert count_characters("hello") == {'h': 1, 'e': 1, 'l': 2, 'o': 1}
  assert count_characters("aaa") == {'a': 3}
output: def count_characters(s):
      frequency = {}
      for char in s:
          if char in frequency:
              frequency[char] += 1
          else:
              frequency[char] = 1
      return frequency