Add typing annotations to Python codes. (#411)

docs/chapter_stack_and_queue/stack.md

@@ -81,7 +81,7 @@
     ```python title="stack.py"
     """ 初始化栈 """
     # Python 没有内置的栈类，可以把 List 当作栈来使用 
-    stack = []
+    stack: List[int] = []
     
     """ 元素入栈 """
     stack.append(1)
@@ -91,16 +91,16 @@
     stack.append(4)
     
     """ 访问栈顶元素 """
-    peek = stack[-1]
+    peek: int = stack[-1]
     
     """ 元素出栈 """
-    pop = stack.pop()
+    pop: int = stack.pop()
     
     """ 获取栈的长度 """
-    size = len(stack)
+    size: int = len(stack)
     
     """ 判断是否为空 """
-    is_empty = len(stack) == 0
+    is_empty: bool = len(stack) == 0
     ```
 
 === "Go"