Fix a potential memory leak in PersistentTaskGroup

[achimnol]

• It no longer stores the unhandled exceptions from subtasks,
  as they will be kept for the whole lifespan of the
  PersistentTaskGroup.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.57 🎉

Comparison is base (540daf2) 74.31% compared to head (0616890) 74.88%.

❗ Current head 0616890 differs from pull request most recent head eb8d186. Consider uploading reports for the commit eb8d186 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   74.31%   74.88%   +0.57%     
==========================================
  Files          17       17              
  Lines        1347     1322      -25     
==========================================
- Hits         1001      990      -11     
+ Misses        346      332      -14     

Impacted Files	Coverage Δ
src/aiotools/server.py	48.98% <ø> (ø)
src/aiotools/func.py	98.03% <100.00%> (+0.03%)	⬆️
src/aiotools/taskgroup/__init__.py	100.00% <100.00%> (ø)
src/aiotools/taskgroup/persistent.py	90.53% <100.00%> (+3.16%)	⬆️
src/aiotools/taskgroup/persistent_compat.py	85.39% <100.00%> (+2.58%)	⬆️
src/aiotools/taskgroup/types.py	90.00% <100.00%> (ø)

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