EHN: let top_n default return is index not tuple

[Zeroto521]

simplify data structure, the tuple is hard to deal.

[codecov]

Codecov Report

Merging #248 (4f3dbf2) into master (9e0a0a7) will decrease coverage by 0.96%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
- Coverage   99.61%   98.65%   -0.97%     
==========================================
  Files          21       21              
  Lines         520      520              
  Branches       55       55              
==========================================
- Hits          518      513       -5     
- Misses          0        3       +3     
- Partials        2        4       +2     

Impacted Files	Coverage Δ
dtoolkit/accessor/dataframe.py	96.15% <ø> (-1.29%)	⬇️
dtoolkit/accessor/_util.py	80.00% <0.00%> (-20.00%)	⬇️

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