Решение задач Code Wars на Python. First non repeating character. 5 kyu

Write a function named first_non_repeating_letter† that takes a string input, and returns the first character that is not repeated anywhere in the string. For example, if given the input ’stress’, the function should return ’t’, since the letter t only occurs once in the string, and occurs first in the string. As an added challenge, upper- and lowercase letters are considered the same character, but the function should return the correct case for the initial letter. For example, the input ’sTreSS’ should return ’T’. If a string contains all repeating characters, it should return an empty string (““); † Note: the function is called firstNonRepeatingLetter for historical reasons, but your function should handle any Unicode character. Разбор задач с Code Wars: Code Wars 8 kyu - Code Wars 7 kyu - Разбор задач по Python (Питон): “Поколение Python“: курс для профессионалов - “Поколение Python“: курс для продвинутых - “Поколение Python“: курс для начинающих - “Программирование на Python“ - Единоразовая поддержка -