Fix deprecation warnings due to invalid escape sequences.

[tirkarthi]

Fixes #3754

[codecov]

Codecov Report

Merging #4924 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4924      +/-   ##
==========================================
+ Coverage   77.10%   77.17%   +0.07%     
==========================================
  Files         128      128              
  Lines       21723    21723              
==========================================
+ Hits        16749    16765      +16     
+ Misses       4974     4958      -16     

Impacted Files	Coverage Δ
src/transformers/tokenization_transfo_xl.py	40.67% <100.00%> (ø)
src/transformers/file_utils.py	73.79% <0.00%> (ø)
src/transformers/modeling_tf_utils.py	87.42% <0.00%> (+0.15%)	⬆️
src/transformers/trainer.py	39.48% <0.00%> (+0.23%)	⬆️
src/transformers/modeling_xlnet.py	78.65% <0.00%> (+2.29%)	⬆️

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[LysandreJik]

LGTM, thanks!