Simplify implementation of state.curdoc

[philippjfr]

So I hadn't realized contextvars.ContextVar was available in Python 3.7 when I wrote the state.curdoc management code. This implementation is much cleaner.

[codecov]

Codecov Report

Merging #3834 (6a4333b) into master (3a2103b) will decrease coverage by 7.62%.
The diff coverage is 66.66%.

❗ Current head 6a4333b differs from pull request most recent head 1188df4. Consider uploading reports for the commit 1188df4 to get more accurate results

@@            Coverage Diff             @@
##           master    #3834      +/-   ##
==========================================
- Coverage   82.89%   75.26%   -7.63%     
==========================================
  Files         222      222              
  Lines       32459    32381      -78     
==========================================
- Hits        26908    24373    -2535     
- Misses       5551     8008    +2457     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	75.26% <66.66%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
panel/io/pyodide.py	0.00% <0.00%> (ø)
panel/io/server.py	74.75% <ø> (-0.20%)	⬇️
panel/tests/conftest.py	80.53% <ø> (-7.68%)	⬇️
panel/io/state.py	68.91% <85.71%> (-2.21%)	⬇️
panel/tests/ui/widgets/test_tabulator.py	9.82% <0.00%> (-89.07%)	⬇️
panel/tests/ui/widgets/test_input.py	11.33% <0.00%> (-88.09%)	⬇️
panel/tests/ui/layout/test_accordion.py	14.72% <0.00%> (-85.28%)	⬇️
panel/tests/ui/widgets/test_sliders.py	15.42% <0.00%> (-84.58%)	⬇️
panel/tests/ui/layout/test_card.py	18.01% <0.00%> (-81.99%)	⬇️
... and 26 more

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