Add doctest

[takenori-y]

#152

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.56%. Comparing base (18edb87) to head (003890b).
⚠️ Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
diffsptk/modules/pitch.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   99.57%   99.56%   -0.01%     
==========================================
  Files         261      261              
  Lines        9492     9505      +13     
==========================================
+ Hits         9452     9464      +12     
- Misses         40       41       +1     

Flag	Coverage Δ
unittests	99.56% <96.42%> (-0.01%)	⬇️

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