* Fix p37-django21-mysql environment not requiring mysqlclient.

* Fix error stating django.contrib.messages must be in INSTALLED_APPS.

* Add py37 to travis file so tests may actually run against it.

So far they seem to be skipped as the interpreter is missing.

* Remove the combination of django21 and sqlite, as travis can't satisfy it.

The problem here is that the Travis tests are based on an image using
Ubuntu Trusty (14.04) which is limited to sqlite3 Version 3.8.2 or below.
There is no easy way to upgrade, it may be simpler (and a better idea) to
upgrade to a newer image, e.g. Xenial (16.04). This is only problematic
with Django 2.1+.

* Remove py37 for now, the trusty image does not support it.

* Remove combination of sqlite and Django 2.1 also from travis file.

I had removed it from the tox file, resulting in this being build in travis
but not doing anything. Better to not even build it.

* Try fixing py34 environments by pinning mysqlclient version.

That version used to work during the last PR on master, so let's see if
it changes something. The 'mysqlclient>=1.3.13' will currently install
version 1.4.2, which may be incompatible with py34.

* Avoid defining mysqlclient dep twice. Limit to below 1.4 to try and fix py34.

* Avoid defining mysqlclient dep twice. Limit to below 1.4 to try and fix py34.

* Order env vars in a way their grouping is more obvious.

* Move JSONField setup to a separate module and support django-mysql on top.

* Fix second import not propery wrapped in try/except block.

* Add a debug statement to check which JSONField was chosen.

* Fix py37 not installing mysqslclient during testruns.

* Install either djangojsonfield or django-mysql depending on tox env.

Also simplify the env setup, so that there is a little less redundancy.

* Check for mysql version (must be 5.7+) before using django-mysql.

* Revert mysql version check in code, as it would need a db connection.

Would otherwise have to rely on some specific 3rd party lib like MySQLdb
or pymysql, which is also no a great idea.

* Make sure the mysql version used for tests is 5.7 to support django-mysql.

* Remove empty line.

* Add mysql version remark.

* Update docs about supported JSONField implementations.

* Fix ambiguity.

* Update docstring about TextField being only a temporary fallback.

* Update changelog.

Add a note about data field being removed if USE_JSONFIELD is not True.

See https://github.com/sphinx-doc/sphinx/issues/3213 for discussion.

Spanish translation

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drop SubQuery as it seems redundant

Add Stream as sponsor

Add Stream as sponsor

added django 2.1 support

Add jsonfield / jsonfield-compat as extras requirement