Add Euronext price source
This adds a price source from euronext.com, which provides the ability to download prices as a .csv file. It does not require an API key for this. This adds quite some tickers that just weren't available before.
Now CI should not fail on linting/typing/testing anymore due to the added/changed files.
@ThomasdenH I believe you can rerun checks and they will succeed now (after #93 was merged)
Tests are still failing due to some typing in an unrelated file, but the PR should be ready now!
Rebased to latest master, CI is passing now. Let me know if anything should be improved!