Remove Dask as a hard dependency of Datashader

[hoxbro]

Resolves #1349

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.15385% with 44 lines in your changes missing coverage. Please review.

Project coverage is 89.89%. Comparing base (93e88ad) to head (726ff84).

Files	Patch %	Lines
datashader/tests/test_raster.py	68.42%	6 Missing :warning:
datashader/bundling.py	44.44%	5 Missing :warning:
datashader/resampling.py	63.63%	4 Missing :warning:
datashader/tests/test_transfer_functions.py	55.55%	4 Missing :warning:
datashader/tests/test_xarray.py	60.00%	4 Missing :warning:
datashader/tiles.py	63.63%	4 Missing :warning:
datashader/core.py	75.00%	3 Missing :warning:
datashader/tests/test_quadmesh.py	76.92%	3 Missing :warning:
datashader/tests/test_dask.py	66.66%	2 Missing :warning:
datashader/tests/test_geopandas.py	50.00%	2 Missing :warning:
... and 4 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1350 +/- ## ========================================== - Coverage 90.05% 89.89% -0.17% ========================================== Files 92 92 Lines 18626 18706 +80 ========================================== + Hits 16774 16815 +41 - Misses 1852 1891 +39 ```

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[hoxbro]

I can see we need to update HoloViews at least in one place:

https://github.com/holoviz/holoviews/blob/eb79ab52414f53b5990f2b624edd4ba582a3fa6e/holoviews/operation/datashader.py#L5