Analyze possibilities to fix attributes typing by implementing mypy plugin

[paveldedik]

Motivation

The biggest issue in Ludic right now is #3. That means, attributes are not type checked, e.g. html(href="/") will pass mypy. It would be nice to check if there is any possibility to fix this temporarily with a mypy plugin - at least until there is a PEP for this and it is implemented by type checkers.