Use yahoo finance api

[LefterisJP]

Abstract

We can add one more oracle source, this one actually also able to query stock/ETF prices.

It's not super stable: https://algotrading101.com/learn/yahoo-finance-api-guide/

But we can use it as long as it works, and it's free. For paid users we could add a premium plan of another API (in the future).

Specification

Think how to achieve this. We don't need to implement the querying ourselves.

There is two python libraries, listed by number of stars:

• https://github.com/ranaroussi/yfinance
• https://github.com/atreadw1492/yahoo_fin

Probably using yfinance would make sense.