Use JSONCodec for json pyaload serialization

codegen/smithy-python-codegen/src/main/java/software/amazon/smithy/python/codegen/SmithyPythonDependency.java

@@ -54,6 +54,16 @@ public final class SmithyPythonDependency {
             List.of("aiohttp")
     );
 
+    /**
+     * The core smithy-json python package.
+     */
+    public static final PythonDependency SMITHY_JSON = new PythonDependency(
+            "smithy_json",
+            "==0.0.1",
+            Type.DEPENDENCY,
+            false
+    );
+
     /**
      * testing framework used in generated functional tests.
      */

codegen/smithy-python-codegen/src/main/java/software/amazon/smithy/python/codegen/generators/ListGenerator.java

@@ -49,7 +49,7 @@ private void generateSerializer() {
                         for e in value:
                             ${?sparse}
                             if e is None:
-                                serializer.write_null(member_schema)
+                                ls.write_null(member_schema)
                             else:
                                 ${3C|}
                             ${/sparse}