add type annotations and pylint configuration

This MR includes

• type annotations (compatible with both Python 2 and 3) for the module
• changes needed to fix bugs highlighted by the type annotations
• changes needed to fix bugs highlighted by pylint
• a pylint config to silence more annoying warnings that we are unlikely to fix

The final code is clean with mypy (type checking) and pylint3 (current upstream release, since the Debian package is not currently installable).