fix: upgrade to pydicom 3

[sammaxwellxyz]

Description

Related issues: #266 https://github.com/pydicom/deid/issues/266

Please include a summary of the change(s) and if relevant, any related issues above. update read_file to dcmread as per suggestion from pydicom 3 release notes https://github.com/pydicom/pydicom/releases/tag/v3.0.0

I couldn't tell if the other changes would cause external breaking changes.

I've run no testing

Checklist

• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] My code follows the style guidelines of this project

Open questions

Questions that require more discussion or to be addressed in future development: The approach as here https://github.com/pydicom/deid/pull/267/files may be worthwhile as well to avoid a similar case in the future