[python] import json encoder only when needed (#5607)

fixes Azure/autorest.python#3005

---------

Co-authored-by: iscai-msft <[email protected]>

Author: iscai-msft

packages/http-client-python/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Bug Fixes
 
+- Only import helpers for serialization if input body is not binary
 - Unify descriptions for credentials in documentation
 
 ### Other Changes

packages/http-client-python/generator/pygen/codegen/models/operation.py

@@ -35,6 +35,7 @@
 )
 from .parameter_list import ParameterList
 from .model_type import ModelType
+from .primitive_types import BinaryType
 from .base import BaseType
 from .combined_type import CombinedType
 from .request_builder import OverloadedRequestBuilder, RequestBuilder
@@ -432,7 +433,8 @@ def imports(  # pylint: disable=too-many-branches, disable=too-many-statements
             file_import.add_submodule_import("typing", "overload", ImportType.STDLIB)
         if self.code_model.options["models_mode"] == "dpg":
             relative_path = self.code_model.get_relative_import_path(serialize_namespace, module_name="_model_base")
-            if self.parameters.has_body:
+            body_param = self.parameters.body_parameter if self.parameters.has_body else None
+            if body_param and not isinstance(body_param.type, BinaryType):
                 if self.has_form_data_body:
                     file_import.add_submodule_import(
                         self.code_model.get_relative_import_path(serialize_namespace), "_model_base", ImportType.LOCAL