Add py.typed marker

[cnaples79]

Summary

• add PEP 561 py.typed marker so type checkers see bundled stubs
• include marker in package_data for distribution

Rationale

• fixes missing typing marker reported in Library Missing Stub File Typed #359 to avoid linter warnings after install

Changes

• add mssql_python/py.typed
• ship py.typed via setup.py package_data

Fixes #359

[sumitmsft]

Hi @cnaples79

Thank you for raising this PR, appreciate your efforts. We will review the PR and share our comments (if any).

Please allow us a few days' time to get back on this. We will assign it to the appropriate engineer by tomorrow.

Regards,
Sumit