Drop unused `TYPE_CHECKING` imports (`Channel`,
`_server`), remove commented-out `import os` in
`_entry.py`, and use `get_runtime_vars()` accessor
instead of bare `_runtime_vars` in `_trio.py`.
Also,
- freshen `__init__.py` layout docstring for the
new per-backend submod structure
- update `_spawn.py` + `_trio.py` module docstrings
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi
f75865fb2e