text: Title: CI: Update type checker (mypy) to match best practices
  Body: Contributors: we need your help with this!  Get started here: https://github.com/cleanlab/cleanlab/issues/407
  
  - [ ] Add strict type annotations for all existing code.
  - [ ]  Switch over CI to use mypy in strict mode (adding [--strict](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-strict)  flag to existing call) once we have full coverage with type annotations.
  
  
  When adding strict type annotations to existing code, you can see functions are currently lacking them by running:
  ```
  mypy --strict --install-types --non-interactive cleanlab
  ```
  
  Helpful discussion here about custom types we want to use/introduce as part of this effort:  https://github.com/cleanlab/cleanlab/pull/317
  