Changes for compatibility with Vega5 and altair 4

[philippjfr]

No description provided.

[codecov]

Codecov Report

Merging #892 into master will increase coverage by <.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #892      +/-   ##
==========================================
+ Coverage    85.1%   85.11%   +<.01%     
==========================================
  Files         104      104              
  Lines       10590    10593       +3     
==========================================
+ Hits         9013     9016       +3     
  Misses       1577     1577

Impacted Files	Coverage Δ
panel/models/vega.py	100% <ø> (ø)	⬆️
panel/pane/vega.py	79.09% <88.88%> (+0.58%)	⬆️

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