prepare CRAN submission for 0.81 release

[hetong007]

@hcho3 I'm bringing changes from #3988 into release_0.81 branch. Let me know if there's other ways to patch.

[codecov-io]

Codecov Report

Merging #3994 into release_0.81 will decrease coverage by 0.19%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##             release_0.81    #3994     +/-   ##
=================================================
- Coverage           51.95%   51.76%   -0.2%     
  Complexity            203      203             
=================================================
  Files                 182      182             
  Lines               14500    14500             
  Branches              495      495             
=================================================
- Hits                 7534     7506     -28     
- Misses               6728     6756     +28     
  Partials              238      238

Impacted Files	Coverage Δ	Complexity Δ
src/common/transform.h	89.28% <ø> (ø)	0 <0> (ø)	⬇️
python-package/xgboost/core.py	77.79% <0%> (-4.45%)	0% <0%> (ø)
python-package/xgboost/compat.py	89.09% <0%> (-1.82%)	0% <0%> (ø)

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[hcho3]

Looks good to me

[hetong007]

Our submission was blocked because it broke a reverse-dependent package.

I have informed the package author at: ymattu/MlBayesOpt#64

Still need to investigate which side is responsible for the fix. I suggest we hold this PR for potential future patches.