Pin Bokeh to <3 in CI

[ianthomas23]

CI runs are picking up Bokeh 3.0 to create examples, this pins Bokeh to < 3.

[codecov[bot]]

Codecov Report

Merging #1150 (b99c822) into master (f2149c2) will increase coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1150      +/-   ##
==========================================
+ Coverage   85.33%   85.37%   +0.04%     
==========================================
  Files          34       34              
  Lines        7781     7782       +1     
==========================================
+ Hits         6640     6644       +4     
+ Misses       1141     1138       -3     

Impacted Files	Coverage Δ
datashader/macros.py	93.00% <0.00%> (+0.07%)	:arrow_up:
datashader/glyphs/line.py	92.80% <0.00%> (+0.19%)	:arrow_up:
datashader/glyphs/polygon.py	94.80% <0.00%> (+0.64%)	:arrow_up:

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