Fix download returns

[yarimiz]

This PR is for fixing 3 issues in the utils.download_returns method:

1. There's no need for progress bar when downloading only 1 ticker
2. There's no need for multithreading when downloading only one ticker. This actually caused issue when running inside Flask
3. Rename the downloaded returns to the ticker's name instead of "Close". This affects the tearsheet.

[grzesir]

Since this library no longer seems to be maintained, we have created a duplicate of it that we plan to maintain and improve over time. If you'd like to use our version check it out here: https://github.com/Lumiwealth/quantstats_lumi

Please feel free to fork and send your pull request to this new library, we will be actively monitoring the library and approving PRs