Taskcluster-taskgraph

Added
- Add ability to run new types of verifications (e.g doc verifications)
- Allow custom `onExitStatus` values for generic-worker based tasks across all platforms

Changed
- Decision docker images updated to Ubuntu 20.04 and Mercurial 5.3.1
- Pass parameters into verifications functions

Fixed
- Don't assume Taskcluster is enabled in generic-worker run-task based tasks
- Exception during local generation when generic-worker tasks exist

Added

- Add pyenv to python docker image

Fixed

- Point generic worker tasks to TC instance instead of proxy URL
- Fixed some tests for local development

Added

- Decision tasks uploads `run-task` and `fetch-content` scripts as artifacts

Fixed

- Forward `description` and `if-dependencies` keys to Task constructor

Added

- An ``--exclude-key`` flag to the CLI for removing subsections of tasks
- Concept of [if-dependencies][0] to optimization logic
- Optional description attribute to `Task` objects

[0]: https://taskcluster-taskgraph.readthedocs.io/en/latest/concepts/task-graphs.html#if-dependencies

Changed

- A minor refactor to optimization logic around logging
- Ability for `run-task` to install Python dependencies after cloning a repo

Fixed

- An exception in default parameters if repo url doesn't contain slashes
- Support `taskgraph test-action-callback` in `run_task` transforms

Fixed

- A bug with custom parameter defaults not being set when running 'taskgraph' locally
- Error in non UTF-8 locales + unicode in commit message
- User specified Taskcluster urls in environment are now normalized

Changed

- Minor refactor to requests session logic in util/taskcluster.py