EHN: to_series could convert two or more columns DataFrame

[Zeroto521]

• closes #xxxx
• whatsnew entry

For two or more columns, now could set index and value to convert a Series

[codecov]

Codecov Report

Merging #468 (eb50cf2) into main (1e7ba30) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #468   +/-   ##
=======================================
  Coverage   99.50%   99.51%           
=======================================
  Files          24       24           
  Lines         612      620    +8     
  Branches       81       85    +4     
=======================================
+ Hits          609      617    +8     
  Misses          1        1           
  Partials        2        2           

Impacted Files	Coverage Δ
dtoolkit/accessor/dataframe.py	98.87% <100.00%> (+0.11%)	⬆️

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