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>

llama_stack/distribution/access_control.py

@@ -4,7 +4,7 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-from typing import Any, Dict, Optional
+from typing import Any
 
 from llama_stack.distribution.datatypes import AccessAttributes
 from llama_stack.log import get_logger
@@ -14,8 +14,8 @@ logger = get_logger(__name__, category="core")
 
 def check_access(
     obj_identifier: str,
-    obj_attributes: Optional[AccessAttributes],
-    user_attributes: Optional[Dict[str, Any]] = None,
+    obj_attributes: AccessAttributes | None,
+    user_attributes: dict[str, Any] | None = None,
 ) -> bool:
     """Check if the current user has access to the given object, based on access attributes.