Added doctest in palindrome.py (#172)

* Added doctest in palindrome.py

* small change to re run lint_python test

strings/python/palindrome.py

@@ -1,10 +1,22 @@
 #!/usr/bin/python3
-# Palindrome Check Function on Python 3
+# Palindrome Check Function using Python 3
 
 # The Palindrome Algorithm
 # this takes in a string and returns a boolean equal to the result of
-# whether the program is a palindrome or not.
+# whether the string is a palindrome or not.
+string_1 = "abba"
+string_2 = "abbcccbba"
+string_3 = "abbccbbba"
+
 def palindrome(s: str) -> bool:
+    """
+    >>> palindrome(string_1)
+    True
+    >>> palindrome(string_2)
+    True
+    >>> palindrome(string_3)
+    False
+    """
     # Reverse string using idiomatic python
     reversed_string = s[::-1]
     # return the answer, by comparing string and its reverse
@@ -22,9 +34,6 @@ def is_palindrome(s: str):
 
 # main program
 if __name__ == "__main__":
-    string_1 = "abba"
-    string_2 = "abbcccbba"
-    string_3 = "abbccbbba"
     is_palindrome(string_1)
     is_palindrome(string_2)
     is_palindrome(string_3)