gh-36902: `src/sage/libs/coxeter3/`: fix doctest warnings
    
Use more block-scope tags to avoid doctest warnings like
```
File "src/sage/libs/coxeter3/coxeter.pyx", line 1198, in
sage.libs.coxeter3.coxeter.CoxGroupIterator.__next__
Warning: Consider using a block-scoped tag by inserting the line 'sage:
# optional - coxeter3' just before this line to avoid repeating the tag
4 times
    from sage.libs.coxeter3.coxeter import get_CoxGroup as CoxGroup,
CoxGroupIterator # optional - coxeter3
```

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URL: #36902
Reported by: David Coudert
Reviewer(s): Matthias Köppe

Author: Release Manager