Add types to panel.io

[philippjfr]

As the title says.

[codecov]

Codecov Report

Merging #3505 (255a7cb) into master (e32471f) will decrease coverage by 0.13%.
The diff coverage is 74.64%.

@@            Coverage Diff             @@
##           master    #3505      +/-   ##
==========================================
- Coverage   82.93%   82.80%   -0.14%     
==========================================
  Files         199      199              
  Lines       27211    27296      +85     
==========================================
+ Hits        22568    22603      +35     
- Misses       4643     4693      +50     

Impacted Files	Coverage Δ
panel/pane/plot.py	58.37% <0.00%> (ø)
panel/reactive.py	76.93% <50.00%> (-0.07%)	⬇️
panel/viewable.py	68.22% <50.00%> (-0.23%)	⬇️
panel/io/model.py	61.32% <64.28%> (-0.14%)	⬇️
panel/io/server.py	76.56% <67.34%> (-1.70%)	⬇️
panel/io/save.py	63.63% <70.00%> (-0.72%)	⬇️
panel/io/notebook.py	50.24% <71.42%> (-1.55%)	⬇️
panel/io/location.py	86.52% <77.27%> (-2.03%)	⬇️
panel/io/state.py	68.50% <79.72%> (-2.10%)	⬇️
panel/template/base.py	77.22% <85.71%> (-0.78%)	⬇️
... and 5 more

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