Start enforcing mypy for new/updated files

Background

We need to start enforcing type hinting using mypy for any new/updated files when running the pipeline's pipeline.

Acceptance Criteria

• A mypy job is created in the .gitlab-ci.yml for this project
• The mypy job fails (soft failure) if any findings are found for any new/updated files

Definition of Done

• The mypy job is running successfully for the branch created for this effort
• Any documentation in the README or contributing guide is updated