Add `py.typed` file

[elricbk]

mypy doesn't want to use type annotations in the package unless py.typed file is provided. See PEP-561. It is easy to fix this for the locally installed package – just touch py.typed in the package folder, but it would be better if this file is shipped alongside the package as typehints are already present inline.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[alexander-akhmetov]

#457