Add type annotations to public API. Fix #172

[thibaudcolas]

Description

Fixes #172. The public Python API is only made up of register_context_modifier, which is already typed correctly, and override_tag, which has a pretty simple signature.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

Suggested CHANGELOG entry:

### Added

- The package is now published with type hints for the public API (`register_context_modifier` and `override_tag`) ([#172](https://github.com/torchbox/django-pattern-library/issues/172), [#189](https://github.com/torchbox/django-pattern-library/pull/189)).