refactor!: Drop pillow as a required dependency

[hoxbro]

I am not completely sure we should drop it, but we should at least postpone the import until it is needed.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.51%. Comparing base (f23de59) to head (eb9c886).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
datashader/tiles.py	25.00%	3 Missing ⚠️
datashader/tests/test_tiles.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1365      +/-   ##
==========================================
- Coverage   88.53%   88.51%   -0.02%     
==========================================
  Files          93       93              
  Lines       18744    18750       +6     
==========================================
+ Hits        16595    16597       +2     
- Misses       2149     2153       +4     

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[jbednar]

Looks great to me. Happy to relax that requirement, as long as we clearly announce the change in the release notes. Thanks!

[amaloney]

@amaloney note to look at dropping pillow or defer importing for now

• note that bokeh depends on pillow

[amaloney]

lgtm