Modernize Python typing across the codebase

Automated via ruff UP006, UP007, UP035, UP045 rules (target: py311):

- Replace `typing.List`, `Dict`, `Tuple`, `Set`, `FrozenSet`, `Type`
  with their built-in equivalents (`list`, `dict`, `tuple`, etc.)
- Replace `typing.Optional[X]` with `X | None`
- Replace `typing.Union[X, Y]` with `X | Y`
- Move `Mapping`, `Sequence`, `Callable`, `Awaitable`,
  `MutableMapping`, `MutableSequence`, `Iterator`, `AsyncIterator`,
  `AsyncGenerator` imports from `typing` to `collections.abc`
- Remove now-unused `typing` imports
- Add `from __future__ import annotations` to 5 files that use
  forward-reference strings in `X | "Y"` annotations

examples/transports/transports-small-webrtc.py

@@ -8,7 +8,6 @@ import argparse
 import asyncio
 import os
 from contextlib import asynccontextmanager
-from typing import Dict
 
 import uvicorn
 from dotenv import load_dotenv
@@ -39,7 +38,7 @@ load_dotenv(override=True)
 app = FastAPI()
 
 # Store connections by pc_id
-pcs_map: Dict[str, SmallWebRTCConnection] = {}
+pcs_map: dict[str, SmallWebRTCConnection] = {}
 
 ice_servers = [
     IceServer(