Write a function, which determines the frequency of letters of a text.
A sorted (frequency) list with the letters and their frequency shall be returned.
Special characters and digits shouldn't be counted.
Example:
string = "Monty Python"
x = letter_frequency(string)
print(x)
Output:
[('o', 2), ('n', 2), ('t', 2), ('y', 2), ('h', 1), ('m', 1), ('p', 1)]