fix issue with beautification

src/cli.py

@@ -92,7 +92,6 @@ def beautify(words: list[str]):
     new_beautiful_string = []
 
     for num, letter in enumerate(message):
-        if letter in string.ascii_letters:
         if num % 2:
             letter = flip_char(letter)
         new_beautiful_string.append(letter)

src/helpers.py

@@ -2,6 +2,9 @@ import string
 
 
 def flip_char(char: str):
+    if char.lower() not in string.ascii_letters:
+        return char
+
     if char in string.ascii_lowercase:
         return string.ascii_uppercase[string.ascii_lowercase.find(char)]
     else: