Cleanup the top __init__ module

[maximlt]

• Remove unused numpy import
• No longer import io to use io.open and instead use the builtin open as these two are the same in Python 3.

[codecov-commenter]

Codecov Report

Merging #5516 (f1b0fdf) into master (e5224c6) will decrease coverage by 0.17%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #5516      +/-   ##
==========================================
- Coverage   88.08%   87.91%   -0.18%     
==========================================
  Files         302      302              
  Lines       62277    61607     -670     
==========================================
- Hits        54859    54163     -696     
- Misses       7418     7444      +26     

Impacted Files	Coverage Δ
holoviews/__init__.py	67.85% <66.66%> (-0.57%)	⬇️
holoviews/core/data/cudf.py	16.31% <0.00%> (-3.69%)	⬇️
holoviews/core/data/spatialpandas_dask.py	79.24% <0.00%> (-3.30%)	⬇️
holoviews/ipython/magics.py	62.60% <0.00%> (-2.15%)	⬇️
holoviews/core/data/ibis.py	77.37% <0.00%> (-2.09%)	⬇️
holoviews/ipython/display_hooks.py	58.63% <0.00%> (-1.67%)	⬇️
holoviews/element/tabular.py	49.18% <0.00%> (-1.62%)	⬇️
...iews/tests/plotting/matplotlib/test_elementplot.py	98.46% <0.00%> (-1.54%)	⬇️
holoviews/core/overlay.py	66.42% <0.00%> (-1.38%)	⬇️
holoviews/plotting/util.py	69.26% <0.00%> (-1.38%)	⬇️
... and 84 more

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[maximlt]

CI failing for unrelated problems, merging.