Stacks: valid parentheses

A stack is a last-in, first-out pile. A Python list is already a stack:

stack = []
stack.append("(")   # push
stack[-1]           # "("   — peek at the top
stack.pop()         # "("   — remove and return the top

Stacks are perfect for matching nested brackets: the most recently opened bracket must be the first one to close. This is LeetCode #20.

Your task

Write is_valid(s) where s contains only the characters ()[]{}. Return True if and only if every bracket is closed by the matching kind in the correct order.

is_valid("()[]{}")   # True
is_valid("(]")       # False  (wrong kind)
is_valid("([)]")     # False  (wrong order)
is_valid("")         # True   (nothing unbalanced)

Hint: push every opening bracket. When you meet a closing bracket, the top of the stack must be its matching opener — otherwise it is invalid. At the end the stack must be empty.