trio.sleep_forever should be NoReturn

[Redoubts]

Fixes #3095

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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	Coverage Δ
src/trio/_tests/test_timeouts.py	98.56% <100.00%> (+0.09%)	⬆️
src/trio/_timeouts.py	100.00% <100.00%> (ø)

[A5rocks]

It's a shame we have to use type ignores but this makes sense to me!

[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.

[CoolCat467]

Looks great

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