Make libraries installable with 9.0
[] https://github.com/wmde/WikibaseDataModelServices
[x] Patched https://github.com/wmde/WikibaseDataModelServices/pull/201
[] Released
[] https://github.com/wmde/WikibaseDataModelSerialization
[x] Patched https://github.com/wmde/WikibaseDataModelSerialization/pull/248
[] Released
[] https://github.com/wmde/WikibaseInternalSerialization
[x] Patched https://github.com/wmde/WikibaseInternalSerialization/commit/f18b6fe390e156a7324d951607d9778bf577b4e5#diff-b5d0ee8c97c7abd7e3fa29b9a27d1780
[] Released
Make extensions compatible with both version 7.0 & 9.0 (this will include pulling in new versions of the above libs):
[] Wikibase
[] WikibaseLexeme
[x] WikibaseQuality (This is no longer deployed)
[] WikibaseQualityConstraints
[] PropertySuggester
[] WikibaseMediaInfo
Bump mediawiki-vendor DM from 7.0 to 9.0 (and other needed libs):
[] mediawiki-vendor
Remove back compt from extensions (make installable with 9.0 only):
[] Wikibase
[] WikibaseLexeme
[x] WikibaseQuality (This is no longer deployed)
[] [WikibaseQualityConstraints](https://gerrit.wikimedia.org/r/472024)
[] PropertySuggester
[] WikibaseMediaInfo