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>

pyproject.toml

@@ -144,6 +144,7 @@ exclude = [
 
 [tool.ruff.lint]
 select = [
+    "UP",      # pyupgrade
     "B",       # flake8-bugbear
     "B9",      # flake8-bugbear subset
     "C",       # comprehensions