mypy: Found module but no type hints or library stubs

[fschuch]

Some problems were found when importing file_storehouse from another application. Code quality check with mypy raises an error:

from file_storehouse.engine import EngineS3
    ^
    error: Skipping analyzing "file_storehouse.key_mapping": found module but no type hints or library stubs

Here are some references to help solving the issue:

• https://stackoverflow.com/questions/68791682/pypi-package-found-module-but-no-type-hints-or-library-stubs
• mypy says "no type hints or library stubs" although it parses package's .pyi python-poetry/poetry#2194
• Poetry 1.1.x breaks mypy in a project with source dependencies python-poetry/poetry#3094