Simplify using custom token classes in serializers (#517)

For most cases this could be done by overriding get_token, which is simple
enough. The exception was TokenRefreshSerializer.validate where the entire
method needed to be copy-pasted to allow using a custom replacement for
RefreshToken. The other cases are changed the same way mainly for
consistency.

rest_framework_simplejwt/serializers.py

@@ -24,6 +24,7 @@ def __init__(self, *args, **kwargs):
 
 class TokenObtainSerializer(serializers.Serializer):
     username_field = get_user_model().USERNAME_FIELD
+    token_class = None
 
     default_error_messages = {
         "no_active_account": _("No active account found with the given credentials")
@@ -57,15 +58,11 @@ def validate(self, attrs):
 
     @classmethod
     def get_token(cls, user):
-        raise NotImplementedError(
-            "Must implement `get_token` method for `TokenObtainSerializer` subclasses"
-        )
+        return cls.token_class.for_user(user)
 
 
 class TokenObtainPairSerializer(TokenObtainSerializer):
-    @classmethod
-    def get_token(cls, user):
-        return RefreshToken.for_user(user)
+    token_class = RefreshToken
 
     def validate(self, attrs):
         data = super().validate(attrs)
@@ -82,9 +79,7 @@ def validate(self, attrs):
 
 
 class TokenObtainSlidingSerializer(TokenObtainSerializer):
-    @classmethod
-    def get_token(cls, user):
-        return SlidingToken.for_user(user)
+    token_class = SlidingToken
 
     def validate(self, attrs):
         data = super().validate(attrs)
@@ -102,9 +97,10 @@ def validate(self, attrs):
 class TokenRefreshSerializer(serializers.Serializer):
     refresh = serializers.CharField()
     access = serializers.CharField(read_only=True)
+    token_class = RefreshToken
 
     def validate(self, attrs):
-        refresh = RefreshToken(attrs["refresh"])
+        refresh = self.token_class(attrs["refresh"])
 
         data = {"access": str(refresh.access_token)}
 
@@ -129,9 +125,10 @@ def validate(self, attrs):
 
 class TokenRefreshSlidingSerializer(serializers.Serializer):
     token = serializers.CharField()
+    token_class = SlidingToken
 
     def validate(self, attrs):
-        token = SlidingToken(attrs["token"])
+        token = self.token_class(attrs["token"])
 
         # Check that the timestamp in the "refresh_exp" claim has not
         # passed
@@ -163,9 +160,10 @@ def validate(self, attrs):
 
 class TokenBlacklistSerializer(serializers.Serializer):
     refresh = serializers.CharField()
+    token_class = RefreshToken
 
     def validate(self, attrs):
-        refresh = RefreshToken(attrs["refresh"])
+        refresh = self.token_class(attrs["refresh"])
         try:
             refresh.blacklist()
         except AttributeError: