Allow IPv6 datagram protocol addresses

[popsiclelmlm]

Summary

• Widen asyncio.DatagramProtocol.datagram_received's addr parameter to use socket._RetAddress.
• Add regression coverage for IPv4 and IPv6 datagram protocol overrides.

Reproduction

• Issue asyncio: DatagramProtocol.datagram_received has incorrect annotation for addr parameter #15169 reports that IPv6 datagram endpoints can pass (host, port, flowinfo, scope_id), but the stub only allowed a two-item address tuple.

Root cause

• DatagramProtocol.datagram_received used a two-item tuple type as a practical approximation even though datagram addresses vary by socket family.
• socket.recvfrom already models returned addresses as _RetAddress, which is broad enough for IPv4, IPv6, and non-IP socket families.

Changes

• Import socket._RetAddress in stdlib/asyncio/protocols.pyi.
• Use _RetAddress for DatagramProtocol.datagram_received.
• Add stdlib/@tests/test_cases/asyncio/check_datagram_protocol.py with IPv4 and IPv6 override examples.

Tests

• PATH="$PWD/.venv/bin:$PATH" pre-commit run --files stdlib/asyncio/protocols.pyi stdlib/@tests/test_cases/asyncio/check_datagram_protocol.py
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python tests/regr_test.py stdlib --platform darwin -p 3.10
• PATH="$PWD/.venv/bin:$PATH" npx pyright@1.1.410 stdlib/@tests/test_cases --pythonversion 3.10 -p pyrightconfig.testcases.json
• PATH="$PWD/.venv/bin:$PATH" npx pyright@1.1.410 stdlib/asyncio --pythonversion 3.10 -p pyrightconfig.stricter.json
• git diff --check

Screenshots/Logs

• Not applicable; stdlib stub/test-only change.
• Note: tests/runtests.py stdlib/asyncio also ran locally. Its pyright/pre-commit phases passed, but stdlib stubtest failed on this local Python 3.14 environment due unrelated runtime availability/difference issues such as missing _tkinter, missing _curses.BUTTON5_*, and annotationlib differences.

AI disclosure

• Prepared with assistance from Codex.

Fixes #15169

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

For performance and other reasons we only use tests in extraordinary or edge cases. These tests basically test whether type checkers process Any correctly. We don't need them.

[srittau]

Using Any here makes sense to me, as this is hard to type correctly, but we should just use it directly than obscuring it by going through the _RetAddress alias:

Suggested change

	def datagram_received(self, data: bytes, addr: _RetAddress) -> None: ...
	# addr varies by socket family and can be a tuple[str, int, int, int] for IPv6,
	# tuple[int, int] for some unusual protocols like socket.AF_NETLINK, or other
	# shapes.
	def datagram_received(self, data: bytes, addr: Any) -> None: ...