pandas: Monthly Revenue by Cohort

You're given a DataFrame of orders with columns user_id, order_date (string YYYY-MM-DD) and amount.

Implement cohort_revenue(df) that returns a DataFrame:

• index: cohort month (YYYY-MM string) — the month of each user's first order
• columns: order month (YYYY-MM string)
• values: total revenue, with missing combinations filled with 0

Rows and columns sorted ascending.