Release patch release 1.3.1

[hcho3]

Closes #6518

@dmlc/xgboost-committer

@hetong007 We should submit this release to retain XGBoost on CRAN.

[codecov-io]

Codecov Report

Merging #6543 (12aac79) into release_1.3.0 (76c3614) will increase coverage by 0.08%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           release_1.3.0    #6543      +/-   ##
=================================================
+ Coverage          79.89%   79.98%   +0.08%     
=================================================
  Files                 12       12              
  Lines               3497     3497              
=================================================
+ Hits                2794     2797       +3     
+ Misses               703      700       -3     

Impacted Files	Coverage Δ
python-package/xgboost/tracker.py	95.11% <0.00%> (+1.12%)	⬆️

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