Add some typing information, or make it consistent.
 Add more docstrings.
 Fix using Skip in switch converter.

 Use ©

Used by other modules; these exports got removed when I replaced an ``import *`` earlier.

In the process:
* Update based on latest xcg/templates/odoo_module.
* Fix existing MailTemplate converter (found by these new tests).

Change get__type__ to return a set of values.
It is defined at the Converter level despite only returning a value for a small number of Converters but it simplifies the code using it to do so that way.

Validation and validator can be set on converters after initialization. Only some converter makes use of those values.
This makes the message_to_odoo_validate method unnecessary, that’s why it has been removed.

 Add tests, correct code so test passes.
Fix issues with some converter.

Split converters in multiple modules
Remove deprecated converters
Move legacy converters in a dedicated module