Add py.typed file to support type checking. (PEP 561)

[w1718w]

Description

This PR adds a py.typed marker file and includes it in the package metadata to support PEP 561. This enables external tools such as mypy to recognize and make use of the inline type hints defined in the ptwt package.

Changes made:

• Added py.typed file to indicate that the package supports type hints.
• Updated setup.cfg to include py.typed under [options.package_data].

Reason

Without the py.typed marker file, tools like mypy cannot detect type hints in this package even though type annotations are present in the source.

[v0lta]

Thanks! This is a good idea.