Enable plotting of PNG images with alpha channel

[ChristianHinge]

Fixes #724

Description

ImagePIL.fromarray accepts and correctly interprets the fourth channel of a png-derived tensor as an alpha channel. I believe that simply allowing for four channels should do the trick. I have tested on a single image with alpha channel.

Checklist

• [x] I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• [x] Non-breaking change (would not break existing functionality)
• [ ] Breaking change (would cause existing functionality to change)
• [ ] Tests added or modified to cover the changes
• [x] Integration tests passed locally by running pytest
• [ ] In-line docstrings updated
• [ ] Documentation updated, tested running make html inside the docs/ folder
• [x] This pull request is ready to be reviewed

[fepegar]

@all-contributors please add @ChristianHinge for bug

[allcontributors[bot]]

@fepegar

I've put up a pull request to add @ChristianHinge! :tada: