Add `throttling_retry` module providing a 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.
I also ensured that introduced code can be executed and tested with the
tenacity version packaged in debian buster (`4.12`).
Example of use:
```lang=python
import requests
from swh.lister.utils import throttling_retry
URL = "http://example.org/api/repositories"
@throttling_retry()
def make_request():
response = requests.get(URL)
response.raise_for_status()
return response
```