Fix Literal import for python37

[dennisbader]

Fixes #1444.

Summary

• fixes Literal import for python3.7

[codecov-commenter]

Codecov Report

Base: 93.94% // Head: 93.92% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (b81db0a) compared to base (c52e9d3).
Patch coverage: 60.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1449      +/-   ##
==========================================
- Coverage   93.94%   93.92%   -0.03%     
==========================================
  Files         122      122              
  Lines       10723    10713      -10     
==========================================
- Hits        10074    10062      -12     
- Misses        649      651       +2     

Impacted Files	Coverage Δ
darts/timeseries.py	91.78% <60.00%> (-0.21%)	⬇️
darts/ad/anomaly_model/filtering_am.py	91.93% <0.00%> (-0.13%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.50% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.27% <0.00%> (-0.01%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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[hrzn]

Thanks!

[hrzn]

This could warrant a new patch release...