Pushapkscript

Removed
* Python 3.5 support

Added
* Python 3.7 support

Changed
* `google_play_track` in task payload can now be a random string. Value is enforced by mozapkpublisher.

Added
* Support for Firefox Focus
* Support for Google Play's new internal track.

Removed
* Removed architecture detection. It's now delegated in mozapkpublisher.

Changed
* Updated mozapkpublisher call to match new function signature

Changed
* `script.async_main()` relies on scriptworker (>= 10.2.0) to:
* initialize context, config, and task
* validate the task schema
* `exceptions.TaskVerificationError` in favor of the one in scriptworker
* `script.usage()` now handled by scriptworker
* `task.validate_task_schema()` now handled by scriptworker

Removed
* Now that Firefox 59 is on release:
* `dry_run` is not accepted anymore in task payload
* strings aren't fetched anymore by this worker type

Added
* Google Play strings are now optionally fetched from an upstream task.

Removed
* Deprecated `payload.dry_run` in favor of `payload.commit` in task definition

Added
* Add support of dep-signing. dep-signing is used by testing APKs. pushapkscript won't make a single request to Google Play if such APK is detected.