Fix SARIMAXModel to work with exogenous data on pmdarima>=2.0

[Mr-Geekman]

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?

Proposed Changes

Look #939.

Closing issues

Closes #939.

[github-actions]

🚀 Deployed on https://deploy-preview-940--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #940 (1a7ce15) into master (cb19d87) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #940   +/-   ##
=======================================
  Coverage   84.99%   85.00%           
=======================================
  Files         133      133           
  Lines        7611     7614    +3     
=======================================
+ Hits         6469     6472    +3     
  Misses       1142     1142           

Impacted Files	Coverage Δ
etna/models/sarimax.py	94.77% <ø> (ø)
etna/libs/pmdarima_utils/arima.py	100.00% <100.00%> (ø)
etna/models/autoarima.py	100.00% <100.00%> (ø)

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[martins0n]

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