dev(hansbug): upgrade keys, values and items for TreeValue class, like that in dict object

[HansBug]

#44

[codecov]

Codecov Report

Merging #48 (01897a2) into main (ae999e1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
+ Coverage   99.22%   99.24%   +0.02%     
==========================================
  Files          35       35              
  Lines        2066     2125      +59     
==========================================
+ Hits         2050     2109      +59     
  Misses         16       16              

Flag	Coverage Δ
unittests	99.24% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
treevalue/tree/common/storage.pyx	100.00% <100.00%> (ø)
treevalue/tree/tree/tree.pyx	100.00% <100.00%> (ø)

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