DeprecationWarning: replace safe_str_cmp with hmac.compare_digest

[federicoemartinez]

When using flask 2.0, we get:
flask_httpauth.py:249: DeprecationWarning: 'safe_str_cmp' is deprecated and will be removed in Werkzeug 2.1. Use 'hmac.compare_digest' instead. safe_str_cmp(client_password, stored_password) else None

This PR performs that change

[miguelgrinberg]

Thanks @federicoemartinez!

[federicoemartinez]

I see it fails for python 2.7.
If I'm not wrong Werkzeug supports Python 3.6 and newer.
How would you like to handle this? I could check the python version and use safe_str_cmp if we are on python 2.7. We could also use six?

[miguelgrinberg]

I removed Python 2 support, no need to keep it at this point. Thanks!