readonly fields in schema are ignored during import

[kittauri]

Describe the bug

When importing data, fields with readonly=True in schema will not be set.

Expected behavior

The importer should set the fields as they were in the old system.

Software (please complete the following information):

• OS: EndavourOS with 6.18 kernel
• Browser 145.0.7632.116 (Offizieller Build) (64-Bit)
• Plone Version 6.1.3
• Volto Version 19.0.0-alpha.19
• Plone REST API Version 9.15.3

[davisagli]

plone.restapi currently skips readonly fields (https://github.com/plone/plone.restapi/blob/main/src/plone/restapi/deserializer/dxcontent.py#L94), which is what we want under normal conditions.

Maybe we can find a way to temporarily enable it while importing. But it will also be blocked when plone.restapi tries to use a z3c.form data manager to store the value (https://github.com/zopefoundation/z3c.form/blob/17c6a5e745a292eaebd026db18d541c13a40345c/src/z3c/form/datamanager.py#L82)

So: we can try to solve this, but it will not be easy.