Drop _type from dataframe

[manycoding]

#104
I doubt that having _type is useful, so I removed it from Items.df
Items.raw is untouched.

[codecov]

Codecov Report

Merging #105 into 0.3.6dev will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           0.3.6dev   #105      +/-   ##
==========================================
+ Coverage     78.98%    79%   +0.01%     
==========================================
  Files            23     23              
  Lines          1599   1600       +1     
  Branches        280    280              
==========================================
+ Hits           1263   1264       +1     
  Misses          290    290              
  Partials         46     46

Impacted Files	Coverage Δ
src/arche/readers/items.py	85.36% <100%> (+0.11%)	⬆️

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[manycoding]

Merged to #106