DOC Fix incorrect source code link

[hasan-yaman]

Reference Issue

What does this implement/fix? Explain your changes.

Some source code links are wrong in the API reference. For example make_imbalance, fetch_datasets, classification_report_imbalanced and many more. The common thing between those objects is they wrapped by a decorator. Similar problem occurred in scikit-learn in the past. This PR fixed it.

Any other comments?

As stated in the PR, I also dropped removed Python 2 related lines because Python 2 support is dropped in 0.5.0. Also, source code links now points to the decorator.

[codecov[bot]]

Codecov Report

Merging #906 (c285a67) into master (6176807) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #906   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files          97       97           
  Lines        6348     6348           
  Branches      711      711           
=======================================
  Hits         6170     6170           
  Misses        106      106           
  Partials       72       72           

Impacted Files	Coverage Δ
imblearn/keras/_generator.py	92.10% <0.00%> (ø)

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[glemaitre]

This is an external extension. We will not touch the code.