Improve hash calculation

[nolze]

Resolves #21.

[codecov-commenter]

Codecov Report

Merging #53 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   87.00%   87.03%   +0.02%     
==========================================
  Files          13       13              
  Lines        1224     1226       +2     
==========================================
+ Hits         1065     1067       +2     
  Misses        159      159              

Impacted Files	Coverage Δ
msoffcrypto/method/ecma376_agile.py	94.50% <100.00%> (+0.12%)	⬆️

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