Omniduct

This is a minor point release with one improvement:
Allow use of dependencies which are not actually installed but simply available on the Python path.

This release exposes the new PySpark client by default through the Duct registry.

This release adds a database backend atop of PySpark.

This release changes the behaviour of AWS logins. In particular, `opinel` is only used if explicitly requested. This makes the behaviour of `omniduct` more predictable compared to direct use of `boto3`, etc.

This release stops passing `'default'` as the default value for `aws_profile` for S3 filesystems, as this breaks default behaviour for AWS logins.

Another relatively small release with minor breaking changes to the API, pushing it up to the next major version.

Breaking changes:
- All `async` keywords have been replaced with `wait`, in order to support Python 3.7.

Features and enhancements:
- Compatibility fixes for Python 2 data encoding in filesystem operations.
- S3 write support, as well as slightly more robust S3 client connections and the option to skip Hadoop artifacts.
- Improved logging and warnings (including support for indeterminate progress bars).
- Pyhive logging of query has been silenced.
- Added thrift dependency which does not seem to be pulled in upstream.

Behind the scenes:
- Stricter code conventions and linting has resulted in cleaner code.