Don't evaluate default_timezone unless needed (#8531)

If you set a custom timezone for a DateTimeField, the function
self.default_timezone() is still called, since fallback params to
getattr are still evaluated.

This rewrites to use hasattr, so the fallback case is only executed if
it will actually be used. If you render a lot of DateTimeFields in a
serializer, the time spent evaluating default_timezone() once for each
of them can accumulate to quite a bit, which is just unused work in the
case where timezone is already specified on the field.

rest_framework/fields.py

@@ -1179,7 +1179,7 @@ class DateTimeField(Field):
         When `self.default_timezone` is `None`, always return naive datetimes.
         When `self.default_timezone` is not `None`, always return aware datetimes.
         """
-        field_timezone = getattr(self, 'timezone', self.default_timezone())
+        field_timezone = self.timezone if hasattr(self, 'timezone') else self.default_timezone()
 
         if field_timezone is not None:
             if timezone.is_aware(value):