Add basic image/heic support

[L3o-pold]

Todo:

• [x] Fix Python 3.5 tests with pyheif https://github.com/david-poirier-csn/pyheif/issues/7
• [x] Improve image_to_jpeg_pyheif method

[L3o-pold]

Is it possible to drop Python 3.5 support?

[inkhey]

Hello, thanks for your contribution to preview_generator.

Is it possible to drop Python 3.5 support?

Not now, we do use debian stretch, so we do need to keep python 3.5 support for preview_generator, but there is no requirement for heic support on python 3.5.

Can you try to make support for heic working only on python 3.6+ ? you can add pytest skipif (https://docs.pytest.org/en/latest/skipping.html#id1) and use check_dependencies method of builder to refuse autoloading of heic builder on python <3.6.

[L3o-pold]

@inkhey hi, yes thanks for your feedback. I'll have a look ASAP

[L3o-pold]

In fact, Imagick should be able to handle HEIC format without using an other python package.

[L3o-pold]

It should be OK now. If IM is built with HEIC support it works as expected.