[py-tx][mlh] Add un-letterboxing reference implementation to Photo Content Type

[Dcallies]

Goal:

Provide a method in Photo content type which, given an image, returns either the same image, or a smaller image that has square black boxes removed from the side.

Example

Take this image:

Given this "letterboxed" version of the image, the return should be the first image.

Demonstrating

Add a new flag to tx hash that will allow preprocessing steps for images

tx hash -s pdq photo original.png
pdq facefacefaceface

tx hash -s pdq photo letterboxed.png
pdq 0000faceface0000

tx hash -s pdq --preprocess=unletterbox letterboxed.png
pdq facefacefaceface 

Implementing

Try to avoid pulling in new dependencies if you can, even though there are a million libraries that might already solve this. Instead, take a look at the image libraries already included, such as Pillow - there is robust documentation.

Testing

Include a unittest that demonstrates the functionality, including edge cases you might consider. Use the bridge mod photos, either algorithmically updating them