Add __repr__ for Pipeline

[julia-shenshina]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Related Issue

Closing issues

[codecov-commenter]

Codecov Report

Merging #151 (c70a8b6) into master (78f20d2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   87.18%   87.18%           
=======================================
  Files          71       71           
  Lines        3105     3106    +1     
=======================================
+ Hits         2707     2708    +1     
  Misses        398      398           

Impacted Files	Coverage Δ
etna/pipeline/pipeline.py	100.00% <100.00%> (ø)

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[martins0n]

👍