Improve datetime handling of sliders

[philippjfr]

Previously the datetime handling of the DateRangeSlider did not correctly roundtrip datetime values unless you were in the UTC timezone. This would cause a bit of jitter because values coming from the frontend would boomerang causing the slider to reset to an earlier state.

[codecov]

Codecov Report

Merging #3276 (0f0d1dc) into master (73e6839) will increase coverage by 0.00%.
The diff coverage is 95.65%.

@@           Coverage Diff           @@
##           master    #3276   +/-   ##
=======================================
  Coverage   83.27%   83.27%           
=======================================
  Files         195      195           
  Lines       26136    26151   +15     
=======================================
+ Hits        21764    21777   +13     
- Misses       4372     4374    +2     

Impacted Files	Coverage Δ
panel/widgets/slider.py	84.16% <95.23%> (+0.43%)	⬆️
panel/util.py	85.09% <100.00%> (+0.11%)	⬆️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/tests/pane/test_plotly.py	98.15% <0.00%> (-0.62%)	⬇️
panel/io/state.py	69.11% <0.00%> (-0.23%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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