chore: enable pyupgrade fixes (#1806)

# What does this PR do?

The goal of this PR is code base modernization.

Schema reflection code needed a minor adjustment to handle UnionTypes
and collections.abc.AsyncIterator. (Both are preferred for latest Python
releases.)

Note to reviewers: almost all changes here are automatically generated
by pyupgrade. Some additional unused imports were cleaned up. The only
change worth of note can be found under `docs/openapi_generator` and
`llama_stack/strong_typing/schema.py` where reflection code was updated
to deal with "newer" types.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

tests/integration/post_training/test_post_training.py

@@ -3,7 +3,6 @@
 #
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
-from typing import List
 
 import pytest
 
@@ -77,7 +76,7 @@ class TestPostTraining:
     async def test_get_training_jobs(self, post_training_stack):
         post_training_impl = post_training_stack
         jobs_list = await post_training_impl.get_training_jobs()
-        assert isinstance(jobs_list, List)
+        assert isinstance(jobs_list, list)
         assert jobs_list[0].job_uuid == "1234"
 
     @pytest.mark.asyncio