Add type annotation to _rename attribute

[MarcSkovMadsen]

Fixes typing errors when trying to inherit from Panel class with _rename attribute.

[codecov]

Codecov Report

Merging #3561 (32fa747) into master (6752cd5) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3561      +/-   ##
==========================================
+ Coverage   82.71%   82.74%   +0.02%     
==========================================
  Files         202      202              
  Lines       27585    27646      +61     
==========================================
+ Hits        22817    22875      +58     
- Misses       4768     4771       +3     

Impacted Files	Coverage Δ
panel/io/location.py	82.97% <100.00%> (ø)
panel/layout/accordion.py	90.00% <100.00%> (+0.20%)	⬆️
panel/layout/base.py	92.21% <100.00%> (ø)
panel/layout/card.py	96.49% <100.00%> (+0.12%)	⬆️
panel/layout/grid.py	75.07% <100.00%> (+0.14%)	⬆️
panel/layout/gridstack.py	54.09% <100.00%> (+1.55%)	⬆️
panel/layout/tabs.py	91.15% <100.00%> (+0.15%)	⬆️
panel/pane/alert.py	100.00% <100.00%> (ø)
panel/pane/deckgl.py	73.75% <100.00%> (+0.37%)	⬆️
panel/pane/echarts.py	78.33% <100.00%> (+0.74%)	⬆️
... and 27 more

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