refactor: Remove direct pandas import

[hoxbro]

For now, to see what it would take not to do any direct imports of pandas.

This is doing the minimum effort. Probably, some of the functions that depend on pandas can be rewritten to not need to depend on it.

Import time comparison with Tuna (python -X importtime -c 'import holoviews' 2> tuna.log && tuna tuna.log):

After:

Before:

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 17 lines in your changes missing coverage. Please review.

Project coverage is 88.79%. Comparing base (01a80f2) to head (bf0402e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/core/util/versions.py	73.68%	5 Missing ⚠️
holoviews/core/data/cudf.py	0.00%	3 Missing ⚠️
holoviews/core/data/pandas.py	90.47%	2 Missing ⚠️
holoviews/core/util/__init__.py	93.33%	2 Missing ⚠️
holoviews/core/util/types.py	96.66%	2 Missing ⚠️
holoviews/element/util.py	66.66%	2 Missing ⚠️
holoviews/element/graphs.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6476   +/-   ##
=======================================
  Coverage   88.79%   88.79%           
=======================================
  Files         323      325    +2     
  Lines       68940    69034   +94     
=======================================
+ Hits        61216    61302   +86     
- Misses       7724     7732    +8     

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