StreamingBody should implement the file interface

[NathanCanine]

If using s3 for a process intermediary to replace the file system, functions that previously took file arguments need to use either local filesystem for temporary storage or intermediary classes are required. Implementing file interface would simplify transition to s3 as a cloud filesystem.

I may look into this as time permits.

[JordonPhillips]

Duplicate of #879

This is definitely something I'd like to get to eventually. We also may need to rework the entire class if we ever want to update our vendored version of requests.