[USER (data scientist)]: Nice! Now, I want to find old buildings that could be renovated for sustainability. What do you think would be a good criterion for that? Please generate a DataFrame identifying buildings eligible for renovation based on their construction year being at or before a calculated threshold, derived from the 'melbourne_housing' dataset, and save it as a pickle file.
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(buildings_to_renovate)

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

---END CODE TEMPLATE---

[YOU (AI assistant)]: