`pydicom.read_file` doesn't exist for pydicom version > 3.0

[pierre-nedelec]

Awesome library!

However, when trying to read RT file, the current implementation relies on pydicom.read_file() which doesn't exist (at least in pydicom version 3+). Should be replaced by pydicom.dcmread() or specify a version of pydicom that has that attribute.