Remove IPython as a dependency

[erik-hasse]

Fixes #1286.

Summary

Removes IPython as a core dependency, which makes darts easier to install and maintain in production environments.

Other Information

[codecov-commenter]

Codecov Report

Base: 93.84% // Head: 93.84% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (c8a58aa) compared to base (31159b3).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1331      +/-   ##
==========================================
- Coverage   93.84%   93.84%   -0.01%     
==========================================
  Files          78       78              
  Lines        8503     8491      -12     
==========================================
- Hits         7980     7968      -12     
  Misses        523      523              

Impacted Files	Coverage Δ
darts/timeseries.py	92.30% <0.00%> (-0.06%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	87.08% <0.00%> (-0.06%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.27% <0.00%> (-0.01%)	⬇️

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[hrzn]

Thanks!