Make PIL an optional dependency

[beat-buesser]

Description

This pull request makes PIL and optional dependency again.

Fixes #2239

Type of change

Please check all relevant options.

• Improvement (non-breaking)
• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

[codecov-commenter]

Codecov Report

Merging #2240 (38618ca) into dev_1.15.1 (4cd5dc0) will increase coverage by 5.69%.
The diff coverage is 100.00%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@              Coverage Diff               @@
##           dev_1.15.1    #2240      +/-   ##
==============================================
+ Coverage       79.92%   85.62%   +5.69%     
==============================================
  Files             308      308              
  Lines           27448    27448              
  Branches         5044     5044              
==============================================
+ Hits            21938    23502    +1564     
+ Misses           4240     2667    -1573     
- Partials         1270     1279       +9     

Files Changed	Coverage Δ
...cks/poisoning/perturbations/image_perturbations.py	93.58% <100.00%> (ø)
art/visualization.py	90.47% <100.00%> (ø)

... and 52 files with indirect coverage changes

[f4str]

This looks good, thanks! I've tested it locally and everything still works without any issues.