Openneuro-py

We've improved handling of connection timeouts, so downloads should work more reliably even on flakey connections.

- Verify file hahses by default whenever possible.
- Several performance improvements.

- Operate smoothly on Python 3.6, and hopefully on windows `CMD` terminals too.

Forgot to bump the version number with `v2020.5`. Otherwise, includes the same changes.

- Enable parallel downloads (defaults to up to 5 concurrent downloads).
- `openneuro.download()` can now be called from Python scripts directly.
- Raise an exception if user `--include`s a path that doesn't exist in the dataset. Previously, we would silently ignore this issue.
- Optimze checks whether we need to resume a download or not.

TSV and JSON files could previously end up corrupted. This problem is now fixed.