Pin dask for rtd builds

[mraspaud]

This PR fixes the rtd builds by pinning the dask version.

This pinning, along with the ones done for CI (#3035), will need to be removed when holoviews is fixed to use dask >= 2025.1

[mraspaud]

According to holoviz/datashader#1390 the upstream fix will be released this week. Is it worth merging this?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.11%. Comparing base (38d106d) to head (75bc5bf).
Report is 55 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3040    +/-   ##
========================================
  Coverage   96.10%   96.11%            
========================================
  Files         381      383     +2     
  Lines       55544    55673   +129     
========================================
+ Hits        53383    53511   +128     
- Misses       2161     2162     +1     

Flag	Coverage Δ
behaviourtests	3.89% <ø> (-0.01%)	⬇️
unittests	96.21% <ø> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 12906523105

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.222%

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Totals
Change from base Build 12905542942:	0.0%
Covered Lines:	53758
Relevant Lines:	55869

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💛 - Coveralls