Enforce is_valid(raise_exception=False) as a keyword-only argument. (#7952)

* make raise_exception a keyword-only argument

* make raise_exception keyword-only in metaclass

Author: ProstoMaxim

rest_framework/serializers.py

@@ -216,7 +216,7 @@ def save(self, **kwargs):
 
         return self.instance
 
-    def is_valid(self, raise_exception=False):
+    def is_valid(self, *, raise_exception=False):
         assert hasattr(self, 'initial_data'), (
             'Cannot call `.is_valid()` as no `data=` keyword argument was '
             'passed when instantiating the serializer instance.'
@@ -735,7 +735,7 @@ def save(self, **kwargs):
 
         return self.instance
 
-    def is_valid(self, raise_exception=False):
+    def is_valid(self, *, raise_exception=False):
         # This implementation is the same as the default,
         # except that we use lists, rather than dicts, as the empty case.
         assert hasattr(self, 'initial_data'), (