ResourceWarning in test_write_timeout[trio] with trio >=0.27

[musicinmybrain]

I can reproduce this in a virtualenv as follows:

$ git clone https://github.com/encode/httpx.git
$ cd httpx
$ python3.13 -m venv _e
$ . _e/bin/activate
(_e) $ pip install -e .[brotli,cli,http2,socks,zstd]
(_e) $ pip install -r requirements.txt
(_e) $ python -m pytest
[…]
============================================= FAILURES ==============================================
_____________________________________ test_write_timeout[trio] ______________________________________

cls = <class '_pytest.runner.CallInfo'>
func = <function call_and_report.<locals>.<lambda> at 0x7f42f37b3f60>, when = 'call'
reraise = (<class '_pytest.outcomes.Exit'>, <class 'KeyboardInterrupt'>)

    @classmethod
    def from_call(
        cls,
        func: Callable[[], TResult],
        when: Literal["collect", "setup", "call", "teardown"],
        reraise: type[BaseException] | tuple[type[BaseException], ...] | None = None,
    ) -> CallInfo[TResult]:
        """Call func, wrapping the result in a CallInfo.

        :param func:
            The function to call. Called without arguments.
        :type func: Callable[[], _pytest.runner.TResult]
        :param when:
            The phase in which the function is called.
        :param reraise:
            Exception or exceptions that shall propagate if raised by the
            function, instead of being wrapped in the CallInfo.
        """
        excinfo = None
        start = timing.time()
        precise_start = timing.perf_counter()
        try:
>           result: TResult | None = func()

_e/lib64/python3.13/site-packages/_pytest/runner.py:341:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
_e/lib64/python3.13/site-packages/_pytest/runner.py:242: in <lambda>
    lambda: runtest_hook(item=item, **kwds), when=when, reraise=reraise
_e/lib64/python3.13/site-packages/pluggy/_hooks.py:513: in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
_e/lib64/python3.13/site-packages/pluggy/_manager.py:120: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
_e/lib64/python3.13/site-packages/_pytest/threadexception.py:92: in pytest_runtest_call
    yield from thread_exception_runtest_hook()
_e/lib64/python3.13/site-packages/_pytest/threadexception.py:68: in thread_exception_runtest_hook
    yield
_e/lib64/python3.13/site-packages/_pytest/unraisableexception.py:95: in pytest_runtest_call
    yield from unraisable_exception_runtest_hook()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def unraisable_exception_runtest_hook() -> Generator[None]:
        with catch_unraisable_exception() as cm:
            try:
                yield
            finally:
                if cm.unraisable:
                    if cm.unraisable.err_msg is not None:
                        err_msg = cm.unraisable.err_msg
                    else:
                        err_msg = "Exception ignored in"
                    msg = f"{err_msg}: {cm.unraisable.object!r}\n\n"
                    msg += "".join(  
                        traceback.format_exception(
                            cm.unraisable.exc_type,
                            cm.unraisable.exc_value,
                            cm.unraisable.exc_traceback,
                        )
                    )
>                   warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))
E                   pytest.PytestUnraisableExceptionWarning: Exception ignored in: <async_generator object ByteStream.__aiter__ at 0x7f4301dd3b80>
E                   
E                   Traceback (most recent call last):
E                     File "/home/ben/src/forks/httpx/_e/lib64/python3.13/site-packages/trio/_core/_asyncgens.py", line 98, in finalizer
E                       warnings.warn(
E                       ~~~~~~~~~~~~~^
E                           f"Async generator {agen_name!r} was garbage collected before it "
E                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E                       ...<5 lines>...
E                           source=agen,
E                           ^^^^^^^^^^^^
E                       )
E                       ^
E                   ResourceWarning: Async generator 'httpx._content.ByteStream.__aiter__' was garbage collected before it had been exhausted. Surround its use in 'async with aclosing(...):' to ensure that it gets cleaned up as soon as you're done using it.

_e/lib64/python3.13/site-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning
--------------------------------------- Captured stdout call ----------------------------------------
INFO:     127.0.0.1:37104 - "PUT /slow_response HTTP/1.1" 200 OK
====================================== short test summary info ======================================
SKIPPED [1] tests/client/test_auth.py:273: netrc files without a password are valid from Python >= 3.11
============================= 1 failed, 1416 passed, 1 skipped in 4.31s =============================

This regression appears to have showed up with trio 0.27, because it goes away if I do pip install 'trio<0.27' to get trio 0.26.2. It is still present if I do pip install --upgrade trio to get trio 0.28.0.