Update new poetry install method

[jdb78]

Description

This PR updates the poetry install method - fixing the CI.

Checklist

• Linked issues (if existing)
• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Make sure to have fun coding!

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Codecov Report

Base: 90.22% // Head: 90.30% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (030d735) compared to base (0e31e52).
Patch coverage: 86.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1126      +/-   ##
==========================================
+ Coverage   90.22%   90.30%   +0.07%     
==========================================
  Files          31       31              
  Lines        4676     4691      +15     
==========================================
+ Hits         4219     4236      +17     
+ Misses        457      455       -2     

Flag	Coverage Δ
cpu	90.30% <86.66%> (+0.07%)	⬆️
pytest	90.30% <86.66%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pytorch_forecasting/metrics/distributions.py	98.98% <ø> (ø)
pytorch_forecasting/metrics/point.py	91.91% <ø> (ø)
pytorch_forecasting/data/encoders.py	87.43% <80.00%> (-0.31%)	⬇️
pytorch_forecasting/data/examples.py	96.66% <100.00%> (ø)
pytorch_forecasting/metrics/base_metrics.py	80.64% <100.00%> (+0.44%)	⬆️
pytorch_forecasting/models/nn/rnn.py	92.40% <0.00%> (+5.06%)	⬆️

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