Add type annotations for MET

[carstenschroeder]

Breaking change

Proposed change

MET: Add Typing Annotations

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[probot-home-assistant]

Hey there @Danielhiversen, @thimic, mind taking a look at this pull request as it has been labeled with an integration (met) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[MartinHjelmare]

Can we completely type one module at a time, perhaps? The current addition doesn't completely type either of the changed modules.

[carstenschroeder]

Can we completely type one module at a time, perhaps? The current addition doesn't completely type either of the changed modules.

done

[MartinHjelmare]

Looks good!