Edx-opaque-keys

* Included test code in the PyPI package so its mixins can be imported into
other code

* Remove to_deprecated_string and from_deprecated_string API methods
* Support OpaqueKeyField lookups by string
* Support more recent versions of the hypothesis Python module
* Support Python 3.6 instead of 3.5

* Remove pytest-django as a dependency of the optional Django support; it's
only needed for testing, not normal usage

* Remove the accidentally included "settings" package from the distribution

* Added OpaqueKeyField and its subclasses, extracted from edx-platform
* Added the "django" installation option to indicate the optional dependency
on the Django package

* Stop an assortment of deprecation warnings by replacing internal usage of
deprecated properties.
* Trailing '/' characters are no longer valid in serialized usage keys.