fix: Feature detection for ExceptionGroup

[achimnol]

• Let MultiError inherit ExceptionGroup instead of BaseExceptionGroup,
  to be consistent with legacy MultiError.

[codecov]

Codecov Report

Merging #42 (7056ef3) into main (23df3eb) will increase coverage by 0.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   65.41%   65.96%   +0.54%     
==========================================
  Files          16       16              
  Lines        1281     1281              
==========================================
+ Hits          838      845       +7     
+ Misses        443      436       -7     

Impacted Files	Coverage Δ
src/aiotools/taskgroup/base_compat.py	83.08% <ø> (ø)
src/aiotools/taskgroup/types.py	90.00% <100.00%> (+17.50%)	⬆️

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