Consider conforming to PEP 561 in order to make type annotations usable

[alehed]

Without this, the type annotations are not that useful when using mypy, as mypy will consider the package untyped and ignore the type annotations.

Conforming to PEP 561 entails:

• moving from a single-file package to a folder with an __init__.py that exports the same interface as the file.
• adding a py.typed file to the folder (and adding include = ["minimalmodbus/py.typed"] to the pyproject.toml)

If you would like me to make a PR with those changes, please let me know.