Python 2.7 compatibility fixes

[philippjfr]

Ran most of the tests and they seem to run okay.

[jlstevens]

Tests are mostly passing though I'm not expecting py2 to pass on CI. As py2 mostly passes locally, I'll merge once the other tests are green.

[codecov-commenter]

Codecov Report

Merging #5298 (995ace3) into branch_1.14.9 (295011c) will increase coverage by 0.00%.
The diff coverage is 88.88%.

@@              Coverage Diff               @@
##           branch_1.14.9    #5298   +/-   ##
==============================================
  Coverage          86.48%   86.48%           
==============================================
  Files                297      297           
  Lines              62804    62809    +5     
==============================================
+ Hits               54315    54323    +8     
+ Misses              8489     8486    -3     

Impacted Files	Coverage Δ
holoviews/core/layout.py	76.40% <0.00%> (ø)
holoviews/core/dimension.py	85.24% <100.00%> (+0.02%)	⬆️
holoviews/core/util.py	85.05% <100.00%> (+0.01%)	⬆️
holoviews/util/transform.py	86.02% <100.00%> (ø)
holoviews/operation/datashader.py	80.08% <0.00%> (+0.32%)	⬆️

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[jlstevens]

3.7 has just passed on OSX and everything except the other OSX jobs have passed. Merging.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.