compat!: Drop Python 3.9

[hoxbro]

Dropping Python 3.9 for the next HoloViews minor release 1.21.0.

Most of the changes are related to B905 (400 occurrences). I have set the default value to None, indicating that I have not decided whether it is True or False.

[codecov]

Codecov Report

Attention: Patch coverage is 85.41667% with 49 lines in your changes missing coverage. Please review.

Project coverage is 88.83%. Comparing base (74fda3f) to head (1a59fbb).

Files with missing lines	Patch %	Lines
holoviews/core/data/__init__.py	54.54%	5 Missing ⚠️
holoviews/core/data/cudf.py	0.00%	3 Missing ⚠️
holoviews/core/util.py	70.00%	3 Missing ⚠️
holoviews/element/raster.py	40.00%	3 Missing ⚠️
holoviews/operation/element.py	66.66%	3 Missing ⚠️
holoviews/plotting/bokeh/annotation.py	40.00%	3 Missing ⚠️
holoviews/plotting/mpl/plot.py	40.00%	3 Missing ⚠️
holoviews/core/data/multipath.py	66.66%	2 Missing ⚠️
holoviews/core/spaces.py	75.00%	2 Missing ⚠️
holoviews/core/traversal.py	66.66%	2 Missing ⚠️
... and 19 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6549      +/-   ##
==========================================
- Coverage   88.85%   88.83%   -0.03%     
==========================================
  Files         324      324              
  Lines       69226    69206      -20     
==========================================
- Hits        61513    61476      -37     
- Misses       7713     7730      +17     

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