implement Event directly with wait_task_rescheduled rather than ParkingLot

[belm0]

raised in #1944

[codecov]

Codecov Report

Merging #1948 (0976de4) into master (52a210f) will decrease coverage by 0.46%.
The diff coverage is 100.00%.

❗ Current head 0976de4 differs from pull request most recent head d7a84ab. Consider uploading reports for the commit d7a84ab to get more accurate results

@@            Coverage Diff             @@
##           master    #1948      +/-   ##
==========================================
- Coverage   99.58%   99.12%   -0.47%     
==========================================
  Files         114      114              
  Lines       14617    14627      +10     
  Branches     1116     1119       +3     
==========================================
- Hits        14557    14499      -58     
- Misses         43       96      +53     
- Partials       17       32      +15     

Impacted Files	Coverage Δ
trio/_sync.py	100.00% <100.00%> (ø)
trio/_subprocess_platform/waitid.py	35.89% <0.00%> (-33.34%)	⬇️
trio/_core/_wakeup_socketpair.py	84.21% <0.00%> (-15.79%)	⬇️
trio/socket.py	92.30% <0.00%> (-7.70%)	⬇️
trio/_core/_multierror.py	95.50% <0.00%> (-4.50%)	⬇️
trio/_subprocess.py	93.23% <0.00%> (-4.35%)	⬇️
trio/_util.py	96.61% <0.00%> (-3.39%)	⬇️
trio/_core/tests/tutil.py	96.61% <0.00%> (-3.39%)	⬇️
trio/_unix_pipes.py	97.72% <0.00%> (-2.28%)	⬇️
trio/_core/tests/test_ki.py	99.13% <0.00%> (-0.87%)	⬇️
... and 7 more

[belm0]

ready to go (unless it needs a newsfragment)

suggest "squash and merge" option for merge