Use updated openslide v4.0.0

[jacob-rosenthal]

A new version of openslide was recently released with some improvements, notably support for DICOM. Maybe pathml support for DICOM could be moved to use openslide instead of the backend we currently have in place

https://github.com/openslide/openslide/releases/tag/v4.0.0

[jamesgwen]

Notes:

• v4.0.0 corresponds to v1.3.0 in openslide-python

• Check if v.1.3.0 or above works with existing code, change if necessary

• Explore and potentially update DICOM backend