Implement libraries.io sourcerank api

[Nekmo]

Describe your suggested feature

Resume

Currently, the project is using libraries.io through scraping, but after several attempts, the user may be banned. As an alternative, we can implement the libraries.io API (sourcerank section).
https://libraries.io/api/

For example:
https://libraries.io/api/pypi/<package_name>/sourcerank?api_key=<api_key>

The code is currently located in the sourcerank.py file. The new logic for using the API can be added to the same file. The new code should only be used if the api_key is provided as an environment variable. Otherwise, the existing logic should be used.
https://github.com/Nekmo/pip-rating/blob/master/pip_rating/sources/sourcerank.py

The environment variable can be LIBRARIESIO_TOKEN, for example. An example using environment variables is available here:
https://github.com/Nekmo/pip-rating/blob/master/pip_rating/sources/sourcecode_page.py

Requirements

The pull request has the following requirements. Remember, if you have any questions, don't hesitate to ask!

• The new code must coexist with the old code. If the token is not provided, the old logic must still work.
• Check the existing code and the classes they inherit from. The information should be stored and returned with the same existing data structure.
• Use Black to format your code. You can automate it using pre-commit.
• Remember the tests and the documentation. They are located in the tests and docs folders respectively.

If you have any questions or need any help, I'm here to help you! Good luck!

Checklist

• I have searched for this issue before posting it and there isn't a duplicate.