feat: use built-in dict instead of OrderedDict on Python >= 3.7

[whtsky]

BREAK CHANGE: the returned object changed from OrderedDict to dict on Python >= 3.7

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 0.72%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   84.61%   85.34%   +0.72%     
==========================================
  Files           1        1              
  Lines         117      116       -1     
==========================================
  Hits           99       99              
+ Misses         18       17       -1

Impacted Files	Coverage Δ
bencoder.pyx	85.34% <66.66%> (+0.72%)	⬆️

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