Nursery.start() doesn't specifically return None

[TeamSpen210]

Noticed this, start() was set to Any | None, but it never specifically returns None, only if you call started() with no parameter on a TaskStatus[None]. If you're using a different task status, forcing None checks is just wrong.

[A5rocks]

Yeah this makes sense. I don't really think it even matters whether it specifically returns None or not; it's just kind of irrelevant. (... if we want people to check, we should return object. If we don't, then Any. Any | something just doesn't really make sense)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00000%. Comparing base (d63f3a9) to head (776843d).
Report is 186 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff                @@
##                 main        #3224    +/-   ##
================================================
  Coverage   100.00000%   100.00000%            
================================================
  Files             124          124            
  Lines           18764        19244   +480     
  Branches         1269         1353    +84     
================================================
+ Hits            18764        19244   +480     

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	100.00000% <ø> (ø)

... and 6 files with indirect coverage changes

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