Implement PersistentTaskGroup

[achimnol]

resolves #29

[codecov]

Codecov Report

Merging #30 (ef1dcc9) into main (3a65b38) will increase coverage by 1.23%.
The diff coverage is 92.20%.

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   77.38%   78.61%   +1.23%     
==========================================
  Files          10       11       +1     
  Lines         849      926      +77     
==========================================
+ Hits          657      728      +71     
- Misses        192      198       +6     

Impacted Files	Coverage Δ
src/aiotools/ptaskgroup.py	92.10% <92.10%> (ø)
src/aiotools/__init__.py	93.75% <100.00%> (+0.41%)	⬆️

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[achimnol]

TODOs:

• result streaming API (just for fun?)
• more detailed logging options
• more tests?
• docs

[achimnol]

I'm going to merge this and follow the future work in new PRs.