Ability to disable trio entirely, event when installed

[Object905]

I was facing this issues
python-trio/trio#2848
benoitc/gunicorn#3093

I've worked around them by uninstalling selenium (because of trio dependency)
Like in this comment python-trio/trio#2848 (comment)
But if I install selenium and it installs trio, then httpx/httpcore starts importing it and I get that module 'select' has no attribute 'epoll'. error when I import httpx, which imports httpcore, which imports trio, which is incompatible.

So, maybe httpcore should have environment variable to explicitly specify that I don't want to use trio ever, even when it's installed.

 File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/dadata/sync.py", line 7, in <module>
    import httpx
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpx/__init__.py", line 2, in <module>
    from ._api import delete, get, head, options, patch, post, put, request, stream
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpx/_api.py", line 6, in <module>
    from ._client import Client
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpx/_client.py", line 32, in <module>
    from ._transports.default import AsyncHTTPTransport, HTTPTransport
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpx/_transports/default.py", line 32, in <module>
    import httpcore
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpcore/__init__.py", line 1, in <module>
    from ._api import request, stream
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpcore/_api.py", line 5, in <module>
    from ._sync.connection_pool import ConnectionPool
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpcore/_sync/__init__.py", line 1, in <module>
    from .connection import HTTPConnection
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpcore/_sync/connection.py", line 12, in <module>
    from .._synchronization import Lock
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/httpcore/_synchronization.py", line 11, in <module>
    import trio
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/trio/__init__.py", line 23, in <module>
    from ._core import TASK_STATUS_IGNORED as TASK_STATUS_IGNORED  # isort: split
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/trio/_core/__init__.py", line 21, in <module>
    from ._local import RunVar, RunVarToken
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/trio/_core/_local.py", line 9, in <module>
    from . import _run
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/trio/_core/_run.py", line 2827, in <module>
    from ._io_epoll import (
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/trio/_core/_io_epoll.py", line 202, in <module>
    class EpollIOManager:
  File "/home/obj/Dev/dom_backend/.venv/lib/python3.12/site-packages/trio/_core/_io_epoll.py", line 203, in EpollIOManager
    _epoll: select.epoll = attrs.Factory(select.epoll)
                                         ^^^^^^^^^^^^
AttributeError: module 'select' has no attribute 'epoll'. Did you mean: 'poll'?

[tomchristie]

Hmmm... We could switch things around so that trio isn't imported unless required, yes.
I think(?) it'd need a bit of a change to the public API, tho.

So instead of all the backends being available at the top-level of the package, they'd instead be available through individual imports...

from httpcore.backends.trio import TrioBackend
from httpcore.backends.anyio import AnyIOBackend
from httpcore.backends.asyncio import AsyncIOBackend

?