[FEATURE] Add py.typed marker for type checkers

[ferdia-sopermaccafraidh-lrn]

The py.typed acts as a marker for type checkers that the type annotations can be used.

This needs to be enabled to take better advantage of type hinting in our own application code.

Ref: https://typing.readthedocs.io/en/latest/spec/distributing.html#packaging-type-information

Package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing. This marker applies recursively: if a top-level package includes it, all its sub-packages MUST support type checking as well.

## Testing

Tested locally by install the learnosity_sdk before and after the py.typed marker within application code and running mypy to ensure it would pick it up

Before change:

Skipping analyzing "learnosity_sdk": module is installed, but missing library stubs or py.typed marker  [import-untyped]
See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 66 source files)

After change:

Success: no issues found in 66 source files

Checklist

• Feature

• Bug

• Security

• Documentation

• ChangeLog.md updated

• Tests added

• All testsuites passed

• make dist completed successfully