Fix floating point error in IntRangeSlider

[hoxbro]

Fixes #6483

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 82.92%. Comparing base (ddedd8d) to head (e4e87b4).

Files	Patch %	Lines
panel/widgets/slider.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6516      +/-   ##
==========================================
- Coverage   82.93%   82.92%   -0.02%     
==========================================
  Files         313      313              
  Lines       45864    45872       +8     
==========================================
+ Hits        38039    38040       +1     
- Misses       7825     7832       +7     

Flag	Coverage Δ
ui-tests	40.09% <70.00%> (-0.01%)	⬇️
unitexamples-tests	71.13% <20.00%> (-0.02%)	⬇️

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