Python is a dynamically-typed language (see: duck typing), but Python 3.5 and pep-484 introduced type hints as a way to annotate Python code with types. These are not enforced by the language at runtime, but they can be checked by third-party libraries such as mypy. The impetus for adding type hints and using mypy is that they generally encourage developers to write saner, simpler code (the flexibility of Python is a double-edged sword). This issue is mostly about good code style and therefore is optional -- it doesn't block CI setup (but once it is complete, the CI can be used to do this type checking automatically).
Python is a dynamically-typed language (see: duck typing), but Python 3.5 and pep-484 introduced type hints as a way to annotate Python code with types. These are not enforced by the language at runtime, but they can be checked by third-party libraries such as mypy. The impetus for adding type hints and using mypy is that they generally encourage developers to write saner, simpler code (the flexibility of Python is a double-edged sword). This issue is mostly about good code style and therefore is optional -- it doesn't block CI setup (but once it is complete, the CI can be used to do this type checking automatically).