Fix append FutureWarning

[alex-hse-repository]

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• 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

closes #751

[github-actions]

🚀 Deployed on https://deploy-preview-764--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #764 (83e1eb1) into master (2b86768) will increase coverage by 33.84%.
The diff coverage is 77.77%.

@@             Coverage Diff             @@
##           master     #764       +/-   ##
===========================================
+ Coverage   49.88%   83.72%   +33.84%     
===========================================
  Files         122      122               
  Lines        6776     6778        +2     
===========================================
+ Hits         3380     5675     +2295     
+ Misses       3396     1103     -2293     

Impacted Files	Coverage Δ
etna/analysis/plotters.py	26.06% <0.00%> (+14.93%)	⬆️
etna/analysis/eda_utils.py	53.06% <100.00%> (+33.43%)	⬆️
etna/pipeline/base.py	94.50% <100.00%> (+9.92%)	⬆️
etna/models/nn/tft.py	100.00% <0.00%> (+3.22%)	⬆️
etna/metrics/base.py	96.87% <0.00%> (+4.68%)	⬆️
etna/models/seasonal_ma.py	97.36% <0.00%> (+5.26%)	⬆️
etna/models/prophet.py	98.80% <0.00%> (+7.14%)	⬆️
etna/transforms/outliers/point_outliers.py	100.00% <0.00%> (+7.31%)	⬆️
etna/ensembles/base.py	100.00% <0.00%> (+7.40%)	⬆️
etna/pipeline/pipeline.py	96.96% <0.00%> (+9.09%)	⬆️
... and 74 more

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[martins0n]

Let's fix this in tests too. We have the same issue with fixtures

[martins0n]

We have multiple issues with fixtures

[martins0n]

👍