Parasolr

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* Dropped support for Python 3.5
* Now tested against Python 3.6, 3.8, Django 2.2—3.1, Solr 6 and Solr 8
* Continuous integration migrated from Travis-CI to GitHub Actions
* bugfix: in some cases, index script was wrongly detecting ModelIndexable
subclasses as abstract and excluding them; this has been corrected
* ModelIndexable now extends ``django.db.models.Model``; existing code
MUST be updated to avoid double-extending Model
* Default index data has been updated to use a dynamic field ``item_type_s`` instead of ``item_type`` so that basic setup does not require customizing the solr schema.
* ``ModelIndexable.get_related_model`` now supports ForeignKey relationships and django-taggit ``TaggableManager`` when identifying depencies for binding signal handlers

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* bugfix: fix regression in SolrQuerySet `get_stats` in 0.6

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* Solr client now escalates 404 errors instead of logging with no exception
* Schema field declarations now support the ``stored`` option
* Schema field type declarations now pass through arbitrary options
* New method ``total_to_index`` on ``parasolr.indexing.Indexable`` to better
support indexing content that is returned as a generator
* Access to expanded results now available on QueryResponse and SolrQuerySet
* SolrQuerySet no longer wraps return results from ``get_stats`` and ``get_facets`` with QueryResponse
* New last-modified view mixin for use with Django views ``parasolr.django.views.SolrLastModifiedMixin``
* New pytest fixture ``mock_solr_queryset`` to generate a Mock SolrQuerySet that simulates the SolrQuerySet fluent interface

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* Only enable pytest plugin when parasolr is in Django installed apps
and a Solr connection is configured

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* Support default option adding fields to solr schema
* Add utility method to convert Solr timestamp to python datetime

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* bugfix: correct queryset highlighting so it actually works
* Revise pytest plugin code to work on non-django projects