Collective.volto.blocksfield

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- Nothing changed yet.

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- Add serializers and deserializer for slate blocks.
[cekk]
- Add indexer for slate blocks.
[cekk]

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- Drop support for Plone 5.2 and collective.dexteritytextindexer; in Plone6
we use core functions in plone.app.dexterity.textindexer
[lucabel]

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- Fix required python version.
[cekk]

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- add check if value is a dict in SearchableText indexer.
[eikichi18]

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- Register standard blocks serializers/deserializer for IDexterityContent. Otherwise they don't work.
[cekk]