Reorganize imports in valid_palindrome.py

Moved the import of `unittest` to the top of the file for better organization and adherence to Python's import conventions. No functional changes were made to the existing code logic.

Author: nicholasadamou

Two Pointers/valid_palindrome.py

@@ -1,5 +1,7 @@
 # https://neetcode.io/problems/is-palindrome
 
+import unittest
+
 # `isalnum()` method returns True if all the characters are alphanumeric, meaning alphabet letter (a-z) and numbers (0-9).
 # `isalpha()` method returns True if all the characters are alphabet letters (a-z).
 # `isdigit()` method returns True if all the characters are digits (0-9).
@@ -15,8 +17,6 @@
 #          ord('0') <= ord(character) <= ord('9')
 #    )
 
-import unittest
-
 
 def is_palindrome(sentence: str) -> bool:
     n = len(sentence)