pwnedpasswords: a Python code to detect pwned passwords

The code is based on https://api.pwnedpasswords.com/ and provides a minimalistic implementation of the discussion in https://www.troyhunt.com/ive-just-launched-pwned-passwords-version-2/

The procedure implemented ensures privacy via k-Anonymity. In other words, the full password (or its hash) never leaves the machine where the code is run. Only the first 5 characters of the hash are transmitted, and the comparison with the full password hash only happens offline.

For more information on how privacy of checked passwords is safeguarded see https://blog.cloudflare.com/validating-leaked-passwords-with-k-anonymity/