input: Write a python function to count the number of times each letter appears in a given sentence.
  assert count_letter_frequency('We are learning Python') == {'W': 1, 'e': 2, 'a': 2, 'r': 2, 'l': 1, 'n': 3, 'i': 1, 'g': 1, 'P': 1, 'y': 1, 't': 1, 'h': 1, 'o': 1}
  assert count_letter_frequency('Hello World') == {'H': 1, 'e': 1, 'l': 3, 'o': 2, 'W': 1, 'r': 1, 'd': 1}
  assert count_letter_frequency('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 count_letter_frequency(sentence):
      frequency = {}
      for char in sentence:
          if char.isalpha():
              if char in frequency:
                  frequency[char] += 1
              else:
                  frequency[char] = 1
      return frequency