[backport] Fix handling of print period in EvaluationMonitor (#6499)

[trivialfis]

No description provided.

[codecov-io]

Codecov Report

Merging #6534 (230f28c) into release_1.3.0 (bce7ca3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@                Coverage Diff                @@
##           release_1.3.0    #6534      +/-   ##
=================================================
+ Coverage          79.99%   80.02%   +0.02%     
=================================================
  Files                 12       12              
  Lines               3494     3494              
=================================================
+ Hits                2795     2796       +1     
+ Misses               699      698       -1     

Impacted Files	Coverage Δ
python-package/xgboost/callback.py	80.04% <100.00%> (+0.24%)	⬆️

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