Add PIL image regression feature

[ClementPinard]

Make the ImageRegressionFixture compatible not only with bytes but also with direct PIL.Image.Image objects.

Most of the time when I use it, I have to convert my images to bytes only for them to be loaded back to PIL.

This PR tries to make it easier for people not familiar with bytes and image conversion

[nicoddemus]

Thanks @ClementPinard, appreciate the PR!

Could you please fix the tests? Thanks!

[nicoddemus]

The parametrization in the test caused the expected files to change... could you fix that? Thanks!

[prusse-martin]

The parametrization in the test caused the expected files to change... could you fix that? Thanks!

Given the image data has not changed a simple image_regression.check(image_data, diff_threshold=1.0, basename='test_image_regression') would possibly work.

[ClementPinard]

The parametrization in the test caused the expected files to change... could you fix that? Thanks!

Given the image data has not changed a simple image_regression.check(image_data, diff_threshold=1.0, basename='test_image_regression') would possibly work.

Thanks ! I initially created two new images, one for each parametrized test, but your solution is indeed much simpler.

[nicoddemus]

Thanks! The Python 3.9 failure is unrelated and will be fixed in a separate PR.