Fixed #31842 -- Added DEFAULT_HASHING_ALGORITHM transitional setting.

It's a transitional setting helpful in migrating multiple instance of
the same project to Django 3.1+.

Thanks Markus Holtermann for the report and review, Florian
Apolloner for the implementation idea and review, and Carlton Gibson
for the review.

django/contrib/auth/tokens.py

@@ -11,11 +11,14 @@ class PasswordResetTokenGenerator:
     reset mechanism.
     """
     key_salt = "django.contrib.auth.tokens.PasswordResetTokenGenerator"
-    algorithm = 'sha256'
+    algorithm = None
     secret = None
 
     def __init__(self):
         self.secret = self.secret or settings.SECRET_KEY
+        # RemovedInDjango40Warning: when the deprecation ends, replace with:
+        # self.algorithm = self.algorithm or 'sha256'
+        self.algorithm = self.algorithm or settings.DEFAULT_HASHING_ALGORITHM
 
     def make_token(self, user):
         """