Make tbptt imports Python 3.10 compatible

[awaelchli]

What does this PR do?

collections.Sequence got moved to collections.abc.Sequence in Python 3.10.
This fixes the code paths for TBPTT when running in Python 3.10.

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cc @Borda @akihironitta

[codecov]

Codecov Report

Merging #13973 (e86b081) into master (eb233ea) will not change coverage.
The diff coverage is 67%.

@@           Coverage Diff           @@
##           master   #13973   +/-   ##
=======================================
  Coverage      76%      76%           
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  Files         335      335           
  Lines       26368    26368           
=======================================
  Hits        20137    20137           
  Misses       6231     6231