input: Write a python function to create a histogram of character frequencies in a string.
  assert histogram('The quick brown fox jumps over the lazy dog.') == {'t': 2, 'h': 2, 'e': 3, '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, 'l': 1, 'a': 1, 'z': 1, 'y': 1, 'g': 1}
  assert histogram('Hello World!') == {'h': 1, 'e': 1, 'l': 3, 'o': 2, 'w': 1, 'r': 1, 'd': 1}
  assert histogram('Python is fun!') == {'p': 1, 'y': 1, 't': 1, 'h': 1, 'o': 1, 'n': 2, 'i': 1, 's': 1, 'f': 1, 'u': 1}
output: def histogram(s):
      s = s.lower()
      freq = {}
      for char in s:
          if char.isalpha():
              if char in freq:
                  freq[char] += 1
              else:
                  freq[char] = 1
      return freq