When changing the type of a column (if size differs for example),
'selection' field should be considered like a 'char' field (since they
are internaly the same column type)
This will fix some migration issues where 'char' fields were correctly
changed but not 'selection,' field.
Use case:
* create a 6.0 db with 'stock' module installed
* 'state' field in 'stock.move' model is of type 'character varying(16)'
* migrate it to 8.0
* 'state' field is still 'character varying(16)' but should normally be
'character varying'
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