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/models/llama/llama3/prompt_templates/tool_response.py

@@ -12,7 +12,6 @@
 # the top-level of this source tree.
 
 import textwrap
-from typing import Optional
 
 from .base import PromptTemplate, PromptTemplateGeneratorBase
 
@@ -21,8 +20,8 @@ class ToolResponseGenerator(PromptTemplateGeneratorBase):
     def gen(
         self,
         status: str,
-        stdout: Optional[str] = None,
-        stderr: Optional[str] = None,
+        stdout: str | None = None,
+        stderr: str | None = None,
     ):
         assert status in [
             "success",