[USER (data scientist)]: Hey, I've got this dataset of affordable housing sales from the past 5 years, and I want to make a heatmap to see the demand by suburb and year. Let's start by calculating the number of affordable properties sold in each suburb for each year. What are the column names for suburb and year in your dataset? Can you help me out? Please generate a DataFrame summarizing the yearly demand for affordable properties sold in each suburb, captured in the 'melbourne_housing' dataset.
My template of code snippet is:

---BEGIN CODE TEMPLATE---

import pandas as pd
import pickle

melbourne_housing = pd.read_csv("melb_data.csv") 

# YOUR SOLUTION BEGIN:
<code1>
[GENERATE YOUR CODE]
</code1>
# YOUR SOLUTION END:

print(suburb_yearly_demand)

# save data
pickle.dump(suburb_yearly_demand,open("./pred_result/suburb_yearly_demand.pkl","wb"))

---END CODE TEMPLATE---

[YOU (AI assistant)]: Of course! Got it! Here's how you can calculate the number of properties sold: 
