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/apis/providers/providers.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, List, Protocol, runtime_checkable
+from typing import Any, Protocol, runtime_checkable
 
 from pydantic import BaseModel
 
@@ -17,12 +17,12 @@ class ProviderInfo(BaseModel):
     api: str
     provider_id: str
     provider_type: str
-    config: Dict[str, Any]
+    config: dict[str, Any]
     health: HealthResponse
 
 
 class ListProvidersResponse(BaseModel):
-    data: List[ProviderInfo]
+    data: list[ProviderInfo]
 
 
 @runtime_checkable