Django 3.0 support (django-tastypie#1603)

* Fix broken imports, some warnings

* Fix tests broken by changed single-quote escaping

* Loosen number of queries assertion to allow less than, as django 3.0 optimized away one query

* Update travis to run against django 3.0

* flake8

* Compat for contextmanager

* contextmanager didn't move, IDE was confused

* Build docs on 2.2 until gdal bug fixed

* Get debug output from travis about currently installed packages

* Fix less-than assertion

* Temporarily skip building docs under django 3.0

* Fix exclusion-by-env line

* Undo attempt to workaround GDAL; exclusion line isn't going to work

* Revert useless exclusion attempt

* Attempt updating gdal from PPA

* Try building on bionic

* Drop 3.5 support for Django 3.0

* Try other specifier for mod_spatialite

* Exclude python 3.5 from django 3.0 matrix

* Fix for force_str vs force_text under python 2

* Missing django for py35-docs

* Fix examples using render_to_response (deprecated) in docs

* Update docs, version, release notes

Fix missing release notes

Bump version number to release to PyPI

Bump version number

Updating for v0.13.3 release.

Updating for v0.13.2 release.

Version bump for v0.13.1.

Release for v0.13.0. Added missing release notes for v0.12.2 (django-…

…tastypie#1345).

Django 1.8 Compatability

Added 0.12.1 release notes.