Switch to using freezegun for tests that depend on date.

``django-phonenumber-field`` supports the same versions of Python and Django that django-oscar supports, so this should be a safe upgrade.
Fixed failing test due to invalid phone number in fixtures.
References #3144

Django 2.0 reached end of life in April 2019.

* Implement basic abstract thumbnails generator backend

* Create `oscar_thumbnail` template tag in `image_tags`

* Add `easy_thumbnails` to `INSTALLED_APPS` in sandbox settings

* Modify `primary_image` method of `Product` model

* Override in templates Sorl's `thumbnail` tag with `oscar_thumbnail` tag

* Check that related module is installed during thumbnail class initialization

* Add params to `original` field of `ProductImageFactory`

* Add media related URLs to test settings

* Add unit tests for thumbnailer backend

* Create test helpers in `test/utils`

These helpers will be used in tests related to thumbnails

* Add tests to check `oscar_thumbnail` template tag

* Add test to check images/thumbnails deleted for deleted product

* Move `sorl-thumbnail` and `easy-thumbnails` to `extras_require`

With this change these packages can be installed as
* `pip install django-oscar[sorl-thumbnail]`
* `pip install django-oscar[easy-thumbnails]`

* Updated "Template tags" section in docs with info about `oscar_thumbnail`

* Describe `OSCAR_THUMBNAILER` setting in "Settings" section of the docs

* Add information about created thumbnail backend to the release notes

* Add Django 2.2 to the Tox environments list.

* Add Django 2.2 to the Travis environment list.

* Mention in the release docs about Django 2.2 support.

* Upgrade to Django 2.2 in the setup requirements.

* Do not decode encoded base64 string, if it is not bytes instance, which changed in Django 2.2.

* Run Travis CI builds on Ubuntu Xenial, which has SQLite 3.8.3, required by Django 2.2.

* Add Python 3.7 to the test matrix.

* Update comment for the Django slugify function call, since it does not return `SafeString` since Django 2.2.

Django 2.1 works fine - the only issue was that more passwords
have been added to the CommonPasswordValidator which caused some tests
to fail. I've removed that validator from the test settings.

Support for the old-style attributes was dropped in django-extra-views 0.11.

Also pin the version as the package does not use semantic versioning.

Fixes #2688.

We cannot rely on this being installed by django-phonenumber-field.
See https://github.com/django-oscar/django-oscar/issues/2681.

Fixes #2681.

Just released, that’s why

v2.0.0a was released recently which we are not yet compatible with.

The blocking dependency (Haystack) now supports Django 2.

Just assume django-haystack uses semver for now.

pytest-warnings is now part of pytest core. See https://docs.pytest.org/en/latest/changelog.html#id71

- Drop support for Django 1.9 (EOL)
- Drop support for Python 3.6 with Django 1.8 to 1.10