MaasConfig and find_device

[gilesknap]

This is a pedantic issue to make the code look neater.

_devices cannot be declared in the dataclass MaasConfig or it corrupts the schema - hence all the lint ignore directives in function find_device.

Possible solutions:

• Change the schema to make a Dict of SwitchDevice instead of list - but that would be a breaking change.
• Convince API Schema to ignore some fields during serialize if possible ?

[gilesknap]

Resolved.

By declaring _devices as a Classvar

_devices: ClassVar[Dict[str, SwitchDevice]] = {}

We avoid it being in the schema.

This makes MaasConfig a singleton which is OK for our use of it. But not ideal since there is no checks on this and the tests will create multiple instances.