Pywttr-models

- Add support for Galician language.

- Use relative imports.

- Raise minimum required Python version from 3.7 to 3.7.2.
- Add docstring to `__init__.py`.
- Add `from __future__ import annotations` to all modules.

- Remove unsupported languages.

- Minor cleanup.

- Drop Python 3.6 support.
- Update dependencies.