Fix regression initializing DiscreteSlider with non-integer value

[philippjfr]

Fixes #3965

[codecov]

Codecov Report

Merging #4009 (0acb130) into master (00f6f33) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4009      +/-   ##
==========================================
- Coverage   83.67%   83.67%   -0.01%     
==========================================
  Files         223      223              
  Lines       33128    33130       +2     
==========================================
+ Hits        27719    27720       +1     
- Misses       5409     5410       +1     

Flag	Coverage Δ
ui-tests	37.57% <0.00%> (-0.02%)	⬇️
unitexamples-tests	74.69% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/tests/widgets/test_player.py	100.00% <100.00%> (ø)
panel/widgets/player.py	79.06% <100.00%> (+0.49%)	⬆️
panel/tests/util.py	88.34% <0.00%> (-0.98%)	⬇️

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