input: Write a python function to calculate the frequency of each character in a given string.
  assert char_frequency("Hello World!") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}
  assert char_frequency("aabbcc") == {'a': 2, 'b': 2, 'c': 2}
  assert char_frequency("abcABC") == {'a': 1, 'b': 1, 'c': 1, 'A': 1, 'B': 1, 'C': 1}
output: def char_frequency(s):
      frequency = {}
      for char in s:
          if char in frequency:
              frequency[char] += 1
          else:
              frequency[char] = 1
      return frequency