Wtfix

**Fixes**

- Avoid `AttributeError` when a `ConnectionError` occurs in the `client_session` app.
- Refactor task cancellation: client should take responsibility for final task cancellation / cleanup instead of the
pipeline. This ensures that the client itself is not also cancelled as part of a pipeline shutdown.
- Only call `super().stop()` after an app has completed all of its own shutdown routines.
- Don't allow misbehaving apps from interrupting a pipeline shutdown. This ensures that the pipeline can always be
shut down, and all outstanding asyncio tasks cancelled, even if one or more apps raise and exceptions while stopping.

**Fixes**

- `client_session`: Don't wait for `writer` to close when shutting down in order to avoid hangs due to network errors.
- Use the recommended `asyncio.create_task` to create new Tasks, which is preferred to `asyncio.ensure_future`.
- Fix issue that caused the `client_session` listener task to hang during shutdown.

**Fixes**

- Fix cancellation of various `asyncio` tasks causing the pipeline to hang during shutdown.

**Enhancements**

- Refactor graceful shutdown of pipeline to cancel all tasks in parallel and log all exceptions.
- Update exception handling routines to reference the standard asyncio exceptions that were moved to the new
`asyncio.exceptions` package in Python 3.8.
- Now requires Python >= 3.8.
- **BREAKING CONFIG CHANGES** (please update your `.env` and settings files in `wtfix.config.settings`):
- Rename `REDIS_URI` config parameter to `REDIS_WTFIX_URI` so that WTFIX can be incorporated into existing
applications without affecting their configuration settings.

**Fixes**

- `SeqNumManagerApp`: wait until a buffered Message has been completely processed before submitting the next one.
- `RedisPubSubApp`: Release Redis connection before attempting to close the Redis pool.

**Enhancements**

- Relax dependency version requirements in setup.py.
- Update dependencies to latest major versions.

**Fixes**

- Re-raise all exceptions that cause the pipeline to terminate abnormally so that they can be reported and dealt
with at the operating system level. Useful if pipeline is being monitored by something like [supervisord](http://supervisord.org).