Braingeneers-smart-open

This release modifies the handling of transport parameters for the S3 back-end in a backwards-incompatible way.
See [the migration docs](MIGRATING_FROM_OLDER_VERSIONS.rst) for details.

- Refactor S3, replace high-level resource/session API with low-level client API (PR [583](https://github.com/RaRe-Technologies/smart_open/pull/583), [mpenkov](https://github.com/mpenkov))
- Fix potential infinite loop when reading from webhdfs (PR [597](https://github.com/RaRe-Technologies/smart_open/pull/597), [traboukos](https://github.com/traboukos))
- Add timeout parameter for http/https (PR [594](https://github.com/RaRe-Technologies/smart_open/pull/594), [dustymugs](https://github.com/dustymugs))
- Remove `tests` directory from package (PR [589](https://github.com/RaRe-Technologies/smart_open/pull/589), [e-nalepa](https://github.com/e-nalepa))

- Support tell() for text mode write on s3/gcs/azure (PR [582](https://github.com/RaRe-Technologies/smart_open/pull/582), [markopy](https://github.com/markopy))
- Implement option to use a custom buffer during S3 writes (PR [547](https://github.com/RaRe-Technologies/smart_open/pull/547), [mpenkov](https://github.com/mpenkov))

- Correctly pass boto3 resource to writers (PR [576](https://github.com/RaRe-Technologies/smart_open/pull/576), [jackluo923](https://github.com/jackluo923))
- Improve robustness of S3 reading (PR [552](https://github.com/RaRe-Technologies/smart_open/pull/552), [mpenkov](https://github.com/mpenkov))
- Replace codecs with TextIOWrapper to fix newline issues when reading text files (PR [578](https://github.com/RaRe-Technologies/smart_open/pull/578), [markopy](https://github.com/markopy))

- Refactor `s3` submodule to minimize resource usage (PR [569](https://github.com/RaRe-Technologies/smart_open/pull/569), [mpenkov](https://github.com/mpenkov))
- Change `download_as_string` to `download_as_bytes` in `gcs` submodule (PR [571](https://github.com/RaRe-Technologies/smart_open/pull/571), [alexandreyc](https://github.com/alexandreyc))

- Exclude `requests` from `install_requires` dependency list.
If you need it, use `pip install smart_open[http]` or `pip install smart_open[webhdfs]`.

- Fix reading empty file or seeking past end of file for s3 backend (PR [549](https://github.com/RaRe-Technologies/smart_open/pull/549), [jcushman](https://github.com/jcushman))
- Fix handling of rt/wt mode when working with gzip compression (PR [559](https://github.com/RaRe-Technologies/smart_open/pull/559), [mpenkov](https://github.com/mpenkov))
- Bump minimum Python version to 3.6 (PR [562](https://github.com/RaRe-Technologies/smart_open/pull/562), [mpenkov](https://github.com/mpenkov))