Write the function longestsubpalindrome(s), that takes a string s and returns the longest palindrome that occurs as consecutive characters (not just letters, but any characters) in s.
so longestsubpalindrome("ab-4-! ") returns "b-4-b".if there is a tie, return the lexicographically larger value -- in python, a string s1 is lexicographically greater than a string s2 if (s1 > s2).
so: longestsubpalindrome("abcbce") returns "cbc", since ("cbc" > "bcb"). note that unlike the previous functions, this function is case-sensitive (so "a" is not treated the same as "a" here).