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>

scripts/distro_codegen.py

@@ -10,9 +10,9 @@ import importlib
 import json
 import subprocess
 import sys
+from collections.abc import Iterable
 from functools import partial
 from pathlib import Path
-from typing import Iterable
 
 from rich.progress import Progress, SpinnerColumn, TextColumn