Last croissants at the bakery

A bakery's online shop keeps its stock in a slow external store (think: a spreadsheet behind an API). The store has two methods:

• store.get(item) -> int: how many of item are left (0 for an item it has never seen).
• store.put(item, count): set the stock of item.

Implement Shop(store) with:

• buy(item, qty) -> bool: if at least qty of item are in stock, take them (lower the stock by qty) and return True. Otherwise change nothing and return False.
• restock(item, qty) -> None: add qty to the stock of item.

Many customers' threads call these at once on the same Shop.

store = ...                  # 3 croissants, 0 baguettes
shop = Shop(store)
shop.buy("croissant", 2)     # True, 1 left
shop.buy("croissant", 2)     # False, still 1 left
shop.restock("baguette", 5)
shop.buy("baguette", 5)      # True, 0 left
# 10 threads each try buy("croissant", 1) on a stock of 4:
#   exactly 4 get True, 6 get False, the stock ends at 0 (never negative)

The trap is check-then-act: if store.get(item) >= qty: store.put(item, store.get(item) - qty). Two customers can both see "1 left" and both walk away with it. The tests' store is slow enough to make that happen.

Constraints: qty >= 1. Different shops over different stores must not block each other. Don't sleep or busy-wait.