Chaquopy: Python for Android

Nov 6, 2018

* The ABI `arm64-v8a` is now supported.
* Python 2 and Python 3.6.3 are no longer included. However, for existing Python 2 users, Chaquopy 4.x will continue to be maintained until the end of 2019.
* `buildPython` must now be at least Python 3.4.
* `minSdkVersion` must now be at least API level 16.
* Fix various intermittent build errors involving `chaquopy_java.jar`.
* If `pkg_resources` is installed in your app, it will now detect all pip-installed packages.