Deprecate Queue.join and Queue.task_done

[njsmith]

See gh-321.

[codecov]

Codecov Report

Merging #322 into master will decrease coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #322     +/-   ##
=========================================
- Coverage   99.24%   99.05%   -0.2%     
=========================================
  Files          84       86      +2     
  Lines       10345    10778    +433     
  Branches      725      830    +105     
=========================================
+ Hits        10267    10676    +409     
- Misses         61       84     +23     
- Partials       17       18      +1

Impacted Files	Coverage Δ
trio/_deprecate.py	100% <100%> (ø)	⬆️
trio/tests/test_sync.py	100% <100%> (ø)	⬆️
trio/tests/test_deprecate.py	100% <100%> (ø)	⬆️
trio/_sync.py	100% <100%> (ø)	⬆️
trio/_core/_io_windows.py	69.43% <0%> (-7.15%)	⬇️
trio/_core/_io_epoll.py	97.53% <0%> (-2.47%)	⬇️
trio/_core/_io_kqueue.py	78.35% <0%> (-1.17%)	⬇️
trio/__init__.py	100% <0%> (ø)	⬆️
trio/_core/_local.py	100% <0%> (ø)	⬆️
trio/_core/_run.py	100% <0%> (ø)	⬆️
... and 9 more

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[njsmith]

Codecov is probably confused because one of the MacOS builds had to be re-run hours after the rest, and in the mean time there was a merge to master that shifted line numbers around. Before that build came in codecov found no coverage changes.