Chaquopy: Python 2 for Android

May 15, 2018

Demo app:
* Target Android Studio 3.1.

SDK:
* Android Gradle plugin version 3.1 is now supported.
* Android Gradle plugin version 2.2 is no longer supported.
* Add Python versions 2.7.15 and 3.6.5, and fix a few lesser-used standard library modules.
* Update to pip version 10.0.1.
* Build reliability fixes, including one for over-strict metadata parsing.
* Further build speed improvements.
* Improve app startup speed where a requirement is reinstalled at the same version as before.