chore: adding py.typed file to iota_sdk python package

[bpshaver]

Description of change

This signals to static type checkers like mypy to use the type hints included in the source code=

Links to any relevant issues

Fixes #1493

Type of change

Choose a type of change, and delete any options that are not relevant.

• Enhancement (a non-breaking change which adds functionality) - Adds support for type checking for downstream users of the iota_sdk Python package

How the change has been tested

Install mypy and run it on a Python file depending on iota_sdk.

Before the change:

$ echo "from iota_sdk import Wallet" > foo.py && mypy foo.py
foo.py:1: error: Skipping analyzing "iota_sdk": module is installed, but missing library stubs or py.typed marker  [import-untyped]
foo.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

$ echo "from iota_sdk import Wallet" > foo.py && mypy foo.py
Success: no issues found in 1 source file

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• [ x ] I have followed the contribution guidelines for this project
• [ x ] I have performed a self-review of my own code

[Thoralf-M]

Thanks!

[thibault-martinez]

Thank you :)