Don't declare pillow dependency

[kousu]

Checklist

GitHub

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
• I've applied the relevant labels to this PR
• I've assigned a reviewer

PR contents

• I've consulted ivadomed's internal developer documentation to ensure my contribution is in line with any relevant design decisions
• I've added relevant tests for my contribution
• I've updated the relevant documentation for my changes, including argparse descriptions, docstrings, and ReadTheDocs tutorial pages

Description

Pillow (or PIL) is a dependency of imageio, not ivadomed: it's only used in one place in our code, and that, just a test.

By dropping it, we can let imageio choose the version it needs without raising the spectre of version conflicts.

Linked issues

Should help avoid the confusion raised in #1181 (comment)

[coveralls]

Pull Request Test Coverage Report for Build 2769958734

• 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 69.148%

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Totals
Change from base Build 2769957094:	0.0%
Covered Lines:	4236
Relevant Lines:	6126

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💛 - Coveralls

[dyt811]

Thanks for digging into this.