trio.sleep_forever should be NoReturn

[Redoubts]

Fixes #3095

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.58%. Comparing base (4e979bf) to head (e5f38f3). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3096 +/- ## ======================================= Coverage 99.58% 99.58% ======================================= Files 121 121 Lines 17992 18002 +10 Branches 3245 3248 +3 ======================================= + Hits 17917 17927 +10 Misses 52 52 Partials 23 23 ``` | [Files with missing lines](https://app.codecov.io/gh/python-trio/trio/pull/3096?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio) | Coverage Δ | | |---|---|---| | [src/trio/\_tests/test\_timeouts.py](https://app.codecov.io/gh/python-trio/trio/pull/3096?src=pr&el=tree&filepath=src%2Ftrio%2F_tests%2Ftest_timeouts.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX3Rlc3RzL3Rlc3RfdGltZW91dHMucHk=) | `98.56% <100.00%> (+0.09%)` | :arrow_up: | | [src/trio/\_timeouts.py](https://app.codecov.io/gh/python-trio/trio/pull/3096?src=pr&el=tree&filepath=src%2Ftrio%2F_timeouts.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX3RpbWVvdXRzLnB5) | `100.00% <100.00%> (ø)` | |

[TeamSpen210]

Well, actually mypy is correct. If someone did trio.lowlevel.reschedule(), sleep_forever() could succeed. That's totally incorrect to do of course. We could just add a raise TrioInternalError("Should never have been rescheduled!") after the await, which will quiet the type error and make sure you can't do that any more.

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