add API to generate leaf exception from exceptongroup

[WindSoilder]

try to resolve for issue #6

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 2.76%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   66.55%   69.32%   +2.76%     
==========================================
  Files          11       11              
  Lines         311      339      +28     
  Branches       28       31       +3     
==========================================
+ Hits          207      235      +28     
  Misses        101      101              
  Partials        3        3

Impacted Files	Coverage Δ
exceptiongroup/_tests/test_tools.py	100% <100%> (ø)	⬆️
exceptiongroup/_tools.py	60.27% <100%> (+4.21%)	⬆️
exceptiongroup/__init__.py	100% <100%> (ø)	⬆️

[njsmith]

Looks sensible to me. I'm not wild about the leaf_exceptions name, but I'm not thinking of anything better either. @oremanj and @1st1, you've both had opinions about things adjacent to this in the past, any thoughts? (You'll probably want to look at #6 for the description of what this is for.)

[oremanj]

I'm also not wild about the name, and think the implementation looks fine. Considering that people will typically invoke this as for subexc in ExceptionGroup.<thing>(exc):, we don't necessarily need a long name. Some ideas that come to mind are expand, flatten, iterate (or maybe iter).

[WindSoilder]

What about walk (the word comes from os.walk)?
And +1 for iterate :)