Avoid multiple extension execution

[philippjfr]

Combined with holoviz/pyviz_comms#87 this will avoid multiple execution of the extension in one cell.

[codecov]

Codecov Report

Merging #3266 (0655b63) into master (11d4637) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3266      +/-   ##
==========================================
- Coverage   83.17%   83.14%   -0.04%     
==========================================
  Files         195      195              
  Lines       26021    26016       -5     
==========================================
- Hits        21642    21630      -12     
- Misses       4379     4386       +7     

Impacted Files	Coverage Δ
panel/config.py	60.23% <0.00%> (ø)
panel/io/callbacks.py	80.67% <0.00%> (-3.37%)	⬇️
panel/io/reload.py	69.23% <0.00%> (-2.20%)	⬇️
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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