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.

• Copyright year bump, spelling corrections, and project automation updates.
• Removal of Travis-CI and Mypy; utilize GitHub Actions for test automation.
• Updated dependencies and minimum Python version requirements.
• Prevent access to "protected" (underscore-prefixed) plugins via attribute access.
• Bypass object protocol access to accommodate decorator magic.
• Improved test coverage and modernized implementations.
• Package-relative path lookup.
• Reverse plugin name lookup with documentation.