HEP1 - Drop support for Python 3.7

[hoxbro]

With Python 3.7's end of life at the end of June and the upcoming release of Holoviews 1.17, it makes sense to have Holoviews support Python 3.8 and up.

Following: HEP1 at holoviz/holoviz#362

[codecov-commenter]

Codecov Report

Merging #5695 (8a7523f) into main (be58b7e) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5695      +/-   ##
==========================================
- Coverage   88.16%   88.14%   -0.03%     
==========================================
  Files         307      307              
  Lines       62839    62839              
==========================================
- Hits        55405    55390      -15     
- Misses       7434     7449      +15     

Flag	Coverage Δ
ui-tests	22.41% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
holoviews/plotting/util.py	68.78% <ø> (ø)
holoviews/tests/plotting/test_plotutils.py	97.19% <ø> (ø)
holoviews/core/data/ibis.py	76.87% <100.00%> (-4.69%)	⬇️

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[philippjfr]

I'd be in favor of doing this after the 1.16 release. There's no pressing need right now since we still support older versions of Bokeh/Panel.

[hoxbro]

The bump is not related to Bokeh/Panel going to 3.8, but that it will become much more time-consuming supporting old versions of Python, especially in the CI. Note that we will still support older versions of Bokeh/Panel with this change.

Python 3.7 is going out of life in two months and in my opinion, this is close enough to stop supporting it. If we had a more consistent release of HoloViews, a reason could be made to wait, but looking at the history of the last two minor releases, a year can easily go by. 1.15 was released on July 7, 2022, and 1.14 was released on December 3, 2020.