REF/API: make construct_array_type a non-classmethod

[jbrockmendel]

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Reproducible Example

NA

Issue Description

It is already a non-classmethod for StringArray, in order to support different storage attributes that are present on the instance but not on the class. We should make this official if, as seems likely, more dtypes are going to become parametrized (see PDEP-13, PDEP-14)

Expected Behavior

NA

Installed Versions

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[jbrockmendel]

cc @jorisvandenbossche i think making this happen would simplify the StrDtype implementation work. Thoughts?