Smart-open

- 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))

This release modifies the behavior of setup.py with respect to dependencies.
Previously, `boto3` and other AWS-related packages were installed by default.
Now, in order to install them, you need to run either:

pip install smart_open[s3]

to install the AWS dependencies only, or

pip install smart_open[all]

to install all dependencies, including AWS, GCS, etc.

- Include S3 dependencies by default, because removing them in the 2.2.0 minor release was a mistake.

This release modifies the behavior of setup.py with respect to dependencies.
Previously, `boto3` and other AWS-related packages were installed by default.
Now, in order to install them, you need to run either:

pip install smart_open[s3]

to install the AWS dependencies only, or

pip install smart_open[all]

to install all dependencies, including AWS, GCS, etc.

Summary of changes:

- Correctly pass `newline` parameter to built-in `open` function (PR [478](https://github.com/RaRe-Technologies/smart_open/pull/478), [burkovae](https://github.com/burkovae))
- Remove boto as a dependency (PR [523](https://github.com/RaRe-Technologies/smart_open/pull/523), [isobit](https://github.com/isobit))
- Performance improvement: avoid redundant GetObject API queries in s3.Reader (PR [495](https://github.com/RaRe-Technologies/smart_open/pull/495), [jcushman](https://github.com/jcushman))
- Support installing smart_open without AWS dependencies (PR [534](https://github.com/RaRe-Technologies/smart_open/pull/534), [justindujardin](https://github.com/justindujardin))
- Take object version into account in `to_boto3` method (PR [539](https://github.com/RaRe-Technologies/smart_open/pull/539), [interpolatio](https://github.com/interpolatio))

Deprecations

Functionality on the left hand side will be removed in future releases.
Use the functions on the right hand side instead.

- `smart_open.s3_iter_bucket` → `smart_open.s3.iter_bucket`

- Bypass unnecessary GCS storage.buckets.get permission (PR [516](https://github.com/RaRe-Technologies/smart_open/pull/516), [gelioz](https://github.com/gelioz))
- Allow SFTP connection with SSH key (PR [522](https://github.com/RaRe-Technologies/smart_open/pull/522), [rostskadat](https://github.com/rostskadat))