Support for alternate s3 endpoints

This release adds:

• Support for alternate s3 endpoints via #25 (thanks @fbkopp !)
• Updated docs
• Updated python support list

and removes:

• Support for python 3.5

Key Upload/Download/Size

This release adds some useful new tools for S3Key:

• S3Key.download() for downloading to a file path or a file-like object
• S3Key.upload() for uploading to a key from a file path or a file-like object
• S3Key.size() or len(S3Key) for getting the size of the object at the given key

as well as adding:

• 100% coverage (minus one line of raising an Exception on other boto Errors)
• more examples in the readme
• python 3.8 testing on Travis-CI

Type Annotations, CI Setup, and more!

This release updates some of the internals of this library. It adds:

• Type annotations
• in operator on S3Bucket
• del operator on S3Bucket[key]
• more documentation

and removes:

• old python version support