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/test_cases/test_case.py

@@ -20,7 +20,7 @@ class TestCase:
         # loading all test cases
         if self._jsonblob == {}:
             for api in self._apis:
-                with open(pathlib.Path(__file__).parent / f"{api}.json", "r") as f:
+                with open(pathlib.Path(__file__).parent / f"{api}.json") as f:
                     coloned = api.replace("/", ":")
                     try:
                         loaded = json.load(f)