Plone.app.querystring

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Bug fixes:

- Ignore unknown ``sort_on`` indexes when parsing a query.
Otherwise you get a ``CatalogError``. [maurits]

- Add Python 2 / 3 compatibility
[pbauer]

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Bug fixes:

- Fix tests with CMF 2.3. [davisagli]

- Fix querybuilder if SearchableText is a dict {'query': '...'}
[tomgross]

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Bug fixes:

- Filter query string from bad term and operator.
[nngu6036]

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Bug fixes:

- remove unittest2 dependency
[kakshay21]

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Bug fixes:

- Do not fail on empty empty or not existing querystrings while upgrading.
[agitator]

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Bug fixes:

- Fix ``QueryBuilder`` with empty queries to return a Batch when requested.
[thet]

- Disable "Subject" ("Tags") as sortable index.
Keyword Indexes are not sortable.
[jensens]