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/providers/utils/kvstore/sqlite/sqlite.py

@@ -6,7 +6,6 @@
 
 import os
 from datetime import datetime
-from typing import List, Optional
 
 import aiosqlite
 
@@ -33,7 +32,7 @@ class SqliteKVStoreImpl(KVStore):
             )
             await db.commit()
 
-    async def set(self, key: str, value: str, expiration: Optional[datetime] = None) -> None:
+    async def set(self, key: str, value: str, expiration: datetime | None = None) -> None:
         async with aiosqlite.connect(self.db_path) as db:
             await db.execute(
                 f"INSERT OR REPLACE INTO {self.table_name} (key, value, expiration) VALUES (?, ?, ?)",
@@ -41,7 +40,7 @@ class SqliteKVStoreImpl(KVStore):
             )
             await db.commit()
 
-    async def get(self, key: str) -> Optional[str]:
+    async def get(self, key: str) -> str | None:
         async with aiosqlite.connect(self.db_path) as db:
             async with db.execute(f"SELECT value, expiration FROM {self.table_name} WHERE key = ?", (key,)) as cursor:
                 row = await cursor.fetchone()
@@ -55,7 +54,7 @@ class SqliteKVStoreImpl(KVStore):
             await db.execute(f"DELETE FROM {self.table_name} WHERE key = ?", (key,))
             await db.commit()
 
-    async def range(self, start_key: str, end_key: str) -> List[str]:
+    async def range(self, start_key: str, end_key: str) -> list[str]:
         async with aiosqlite.connect(self.db_path) as db:
             async with db.execute(
                 f"SELECT key, value, expiration FROM {self.table_name} WHERE key >= ? AND key <= ?",