Document that Django 2.1 dropped compatbility with py-bcrypt

[Jens-Wolfhard Schicke-Uffmann]

The BCryptSHA256PasswordHasher tries to decode the result of bcrypt.hashpw, which however is already a str (and not bytes).

#!/usr/bin/env python

from django.contrib.auth.hashers import BCryptSHA256PasswordHasher

hasher = BCryptSHA256PasswordHasher()
hasher.encode('secret', hasher.salt())

results in

Traceback (most recent call last):
  File "issue.py", line 6, in <module>
    hasher.encode('secret', hasher.salt())
  File "/mnt/crypt/drahflow/.virtualenvs/NDA/lib/python3.6/site-packages/django/contrib/auth/hashers.py", line 417, in encode
    return "%s$%s" % (self.algorithm, data.decode('ascii'))
AttributeError: 'str' object has no attribute 'decode'

The bug was introduced in: ​https://github.com/django/django/commit/16c5a334ff3ad9d8b3cd1314562c7af20a2a7c7d
Other hashers might be affected, I didn't check.

[Tim Graham]

What version of bcrypt are you using? I don't see the crash with bcrypt 3.1.4 as bcrypt.hashpw() returns a bytestring.

[Jens-Wolfhard Schicke-Uffmann]

Name: py-bcrypt
Version: 0.4
Summary: bcrypt password hashing and key derivation
Home-page: ​https://code.google.com/p/py-bcrypt

[Jens-Wolfhard Schicke-Uffmann]

Not my choice of bcrypt implementation. The project where I observed the problem is ancient and had it lying around in requirements.txt. Switching to bcrypt 3.1.4 solves the problem for me.

Not sure if it's worthwhile to support a seamless upgrade from Django 2.0.7 to Django 2.1 for the other implementation.

[Tim Graham]

Django 1.9 also broke compatibility with py-bcrypt in #26016. At the time (3 years ago), we restored compatibility but I don't see a need to do so at this point since it looks like py-bcrypt is unmaintained. I'll document that in the release notes.

[Tim Graham <timograham@…>]

In 2fa3671:

Fixed #29652 -- Doc'd removal of py-bcrypt compatibility.

[Tim Graham <timograham@…>]

In 548a2d3c:

[2.1.x] Fixed #29652 -- Doc'd removal of py-bcrypt compatibility.

Backport of 2fa36719a8fb39f2c2162f9e6430db00c6bc8412 from master

[Tim Graham <timograham@…>]

In d0928d64:

Refs #29652 -- Fixed typo in docs/releases/2.1.txt.

[Tim Graham <timograham@…>]

In 42f194b:

[2.1.x] Refs #29652 -- Fixed typo in docs/releases/2.1.txt.

Backport of d0928d6454d0b649663820e45d915016a2d9b001 from master