Incorporate downstream CEGID changes.

[amcgregor]

Care of CEGID, my current employer, there are a collection of downstream changes utilized by the RITA job offer multi-posting platform. This pull request aims to homogenize the changes, eliminate the need to pin specific branches instead of released versions, and generally improve the project.

• An overall modernization of Marrow Tags for newer Python 3 versions, including type annotation and namespace packaging.
• Elimination of dependency on long-deprecated marrow.util package.
• Code formatting corrections.