Add swh.lister.utils.throttling_retry decorator enabling to retry a
function that performs an HTTP request who can return a 429 status code.
The implementation is based on the tenacity module and it is assumed
that the requests library is used when querying an URL.
The default wait strategy is based on exponential backoff.
The default max number of attempts is set to 5, HTTPError exception
will then be reraised.
All tenacity.retry parameters can also be overridden in client code.
Streamline production of new listers by aggressively moving core
functionality into progressively inherited (A->B->C) base classes
with the transport layer abstracted.
This should make common individual forge listers straightforward to
produce with minimal customization. Github and Bitbucket listers
can be used as examples of the indexing type.
