Add naturalsquares sequence

sequences.py

 import math
+import helpers
 
 def fibonacci(n):
     """return frist n elements of the fibonacci sequence with seeds 0 and 1"""
@@ -6,3 +7,7 @@ def fibonacci(n):
     for i in range(n-3):
         res.append(res[-1] + res[-2])
     return res
+
+
+def naturalsquares(n):
+    return [x**2 for x in range(n)]