Flask~=1.1; removes some warnings.

[jaheba]

Issue #, if available:

Description of changes:

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[szha]

Job PR-307/1 is complete.
Docs are uploaded to http://gluon-ts-staging.s3-accelerate.dualstack.amazonaws.com/PR-307/1/index.html

[codecov-io]

Codecov Report

Merging #307 into master will increase coverage by 40.06%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #307       +/-   ##
===========================================
+ Coverage   40.23%   80.29%   +40.06%     
===========================================
  Files         142      142               
  Lines        7934     7934               
===========================================
+ Hits         3192     6371     +3179     
+ Misses       4742     1563     -3179

Impacted Files	Coverage Δ
src/gluonts/dataset/artificial/_base.py	36.67% <0%> (+1.43%)	⬆️
src/gluonts/core/exception.py	62.5% <0%> (+2.08%)	⬆️
src/gluonts/model/trivial/constant.py	76.92% <0%> (+11.53%)	⬆️
src/gluonts/kernels/_kernel_output.py	87.09% <0%> (+12.9%)	⬆️
src/gluonts/model/seasonal_naive/_estimator.py	100% <0%> (+14.28%)	⬆️
src/gluonts/model/npts/_estimator.py	100% <0%> (+14.28%)	⬆️
src/gluonts/dataset/artificial/recipe.py	89.21% <0%> (+14.93%)	⬆️
src/gluonts/model/deepar/_estimator.py	100% <0%> (+15.09%)	⬆️
src/gluonts/dataset/stat.py	96.32% <0%> (+15.44%)	⬆️
src/gluonts/block/enc2dec.py	91.66% <0%> (+16.66%)	⬆️
... and 92 more