Each City's Best Seller

The bakery wants to put its most profitable product in each shop window. The orders are split across months, store only a product number, and the names and prices are in a separate product list.

Task: write best_sellers(months, products).

• months is a dictionary from a month's name to that month's orders. Each month's orders are a dictionary of three lists: "city", "product_id" and "qty". Every month has at least one order.
• products is a dictionary of three lists: "product_id", "name" and "price". Every product_id used in an order is in it, once.
• An order's revenue is its qty times its product's price.
• For each city, add up the revenue of each product name across all months.
• Return a dictionary from each city to the name of the product that earned that city the most.
• If two products tie for a city, pick the name that comes first alphabetically. A tie means the two totals are equal once rounded to 2 decimal places.