MAINT: call method Series.get_attr via function name is ok

[Zeroto521]

• closes EHN: new accessor method get_attr #379 and EHN: let register_*_method support alias feature #392
• whatsnew entry

Prefer to call Series.getattr() rather then Series.get_attr(), but the function name is get_attr. It would be a little weird.
So call Series.getattr() is ok, but not recommended.

[codecov]

Codecov Report

Merging #398 (c8c2c82) into master (39d4d50) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #398   +/-   ##
=======================================
  Coverage   99.81%   99.81%           
=======================================
  Files          23       23           
  Lines         550      551    +1     
  Branches       74       74           
=======================================
+ Hits          549      550    +1     
  Partials        1        1           

Impacted Files	Coverage Δ
dtoolkit/accessor/series.py	100.00% <100.00%> (ø)

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