Remove code examples for Python 3.8 in `middleware`

docs/en/docs/tutorial/middleware.md

@@ -31,7 +31,7 @@ The middleware function receives:
     * Then it returns the `response` generated by the corresponding *path operation*.
 * You can then further modify the `response` before returning it.
 
-{* ../../docs_src/middleware/tutorial001.py hl[8:9,11,14] *}
+{* ../../docs_src/middleware/tutorial001_py39.py hl[8:9,11,14] *}
 
 /// tip
 
@@ -57,7 +57,7 @@ And also after the `response` is generated, before returning it.
 
 For example, you could add a custom header `X-Process-Time` containing the time in seconds that it took to process the request and generate a response:
 
-{* ../../docs_src/middleware/tutorial001.py hl[10,12:13] *}
+{* ../../docs_src/middleware/tutorial001_py39.py hl[10,12:13] *}
 
 /// tip