feat: adds single_number exercise

Author: flamiedev

single_number.py

@@ -0,0 +1,11 @@
+from typing import List
+
+
+class Solution:
+    def singleNumber(self, nums: List[int]) -> int:
+        result = 0
+
+        for num in nums:
+            result ^= num
+
+        return result