Remove local functionalities implemted in Orange

[PrimozGodec]

Issue

After the new year, two tests started to warn that changes could be made

Description of changes

• Commit 1: In Corpus, copying of attributes dictionary is no longer required since it is now implemented in Orange [FIX] Table - copy attributes when creating table from table orange3#6189
• Commit 2: enum2int is also implemented in Orange >=3.35 and can be used from there; hence, the local implementation is removed
• Commit 3: When I removed copying of the attributes dict, I realized retain_preprocessing is also copying this dict. Since it is handled by Orange, I removed copying in retain_preprocessing.

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #1031 (f519388) into master (02b1892) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1031      +/-   ##
==========================================
- Coverage   82.20%   82.16%   -0.05%     
==========================================
  Files          93       92       -1     
  Lines       12295    12283      -12     
  Branches     1670     1670              
==========================================
- Hits        10107    10092      -15     
- Misses       1880     1882       +2     
- Partials      308      309       +1