Test suite clean up and fix for the pip build

[maximlt]

This turned into a bigger PR than I thought. My original intention was to fix the pip build that appeared to be broken when it last run on the scheduled jobs. I saw that the grouped dependencies in setup.py where not so clear, so started to re-organize them and checked in particular the ones required to run the tests (not the notebooks). That led me to simplify a lot of imports in the tests. For instance in many places pandas was assumed to be optional while it's now a required dependency of HoloViews, same for Panel. The tests also now need Bokeh, Matplotlib and Plotly to be installed to run. This made sense to me, they're so critical to HoloViews.

[codecov-commenter]

Codecov Report

Merging #5326 (963492f) into master (7d57441) will increase coverage by 1.36%.
The diff coverage is 94.31%.

@@            Coverage Diff             @@
##           master    #5326      +/-   ##
==========================================
+ Coverage   86.64%   88.00%   +1.36%     
==========================================
  Files         299      299              
  Lines       62227    61849     -378     
==========================================
+ Hits        53917    54432     +515     
+ Misses       8310     7417     -893     

Impacted Files	Coverage Δ
holoviews/tests/core/test_dynamic.py	99.44% <ø> (+0.10%)	⬆️
...oloviews/tests/plotting/bokeh/test_hextilesplot.py	100.00% <ø> (+2.22%)	⬆️
holoviews/core/util.py	86.00% <61.53%> (+0.11%)	⬆️
holoviews/tests/core/data/base.py	100.00% <100.00%> (+0.53%)	⬆️
holoviews/tests/core/data/test_daskinterface.py	86.66% <100.00%> (ø)
holoviews/tests/core/data/test_gridinterface.py	99.40% <100.00%> (-0.01%)	⬇️
holoviews/tests/core/data/test_multiinterface.py	99.41% <100.00%> (+0.85%)	⬆️
holoviews/tests/core/data/test_pandasinterface.py	100.00% <100.00%> (+1.58%)	⬆️
holoviews/tests/core/data/test_xarrayinterface.py	98.78% <100.00%> (ø)
holoviews/tests/core/test_dimensions.py	99.52% <100.00%> (+2.68%)	⬆️
... and 63 more

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[maximlt]

I've manually launched the docs and the packages workflows in dry-run mode and they succeeded, merging then.

[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.