Please help with this python 3 programming question

Using python 3 and the function max_relative_frequency(s) which takes as argument a string and returns the highest relative frequency of any letter in the string. This value is unique. There can be several letters that occur with an equal (highest) frequency, but in that case their relative frequency is the same.

• Consider only letters of the English alphabet (that is, 'a' through 'z').
• For the purpose of counting occurrances, consider letters that differ only by case to be the same letter. For example, the letter 'n' occurs three times in the string "Non-even" (once in upper case, twice in lower case).
• For the purpose of counting the total number of letters in the string, count only letters. For example, the string '0 << c++ << 9' contains only one letter, 'c', so the relative frequency of 'c' in this string is 1.
• If the string contains no letter, the relative frequency is undefined and your function should return 0.