Signingscript

Added
- Added aiohttp3 support

Changed
- Dropped py35 support; added py37 tests
- Packaged README.md

Removed
- Removed README.rst

Added
- Added focus-jar support

Added
- Create `KEY` artifact when doing GPG signing

Changed
- `script.async_main()` relies on scriptworker (>= 10.2.0) to:
- initialize context, config, and task
- validate the task schema

Removed
- `exceptions.TaskVerificationError` in favor of the one in scriptworker
- `script.SigningContext`, `script.usage()`, `script.main()` now handled by scriptworker
- `task.validate_task_schema()` now handled by scriptworker

Added
- support for different scope prefix (was hardcoded to `project:releng:signing:`). Prefixes are now defined in the configuration under `taskcluster_scope_prefix`.
- certificates of MDC1 datacenter.

Changed
- `task_cert_type()` now lives under the `task` module.

Added
- added additional dependency for datadog statsd