build(deps): pin opencv-python for linux compatibility (Unstructured-…

…IO#18)

chore: cut release for image processing (Unstructured-IO#13)

- Added is_image flag for process_data_with_model and process_file_with_model
- Added support for the above in the API
- Added testing for all of the above

chore: remove detectron from setup.py (Unstructured-IO#6)

Having a non-pip dependency prevents unstructured-inference from being installed as a package from pypi, so this PR removes detectron2 from the setup.py dependencies. Also removed the code to fail the install if torch isn't installed - torch will be installed with layoutparser, and will be present when the user builds Detectron2 from source.

Public release