Add is_secret to KeyInterface.

[dajiaji]

No description provided.

[codecov]

Codecov Report

Merging #82 (ef0e350) into main (dd78886) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   98.61%   98.62%   +0.01%     
==========================================
  Files          14       14              
  Lines        1301     1313      +12     
  Branches      192      196       +4     
==========================================
+ Hits         1283     1295      +12     
  Misses         17       17              
  Partials        1        1              

Impacted Files	Coverage Δ
pyseto/key_interface.py	100.00% <100.00%> (ø)
pyseto/versions/v1.py	98.66% <100.00%> (+0.01%)	⬆️
pyseto/versions/v2.py	95.87% <100.00%> (+0.08%)	⬆️
pyseto/versions/v3.py	96.73% <100.00%> (+0.03%)	⬆️
pyseto/versions/v4.py	97.75% <100.00%> (+0.05%)	⬆️

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