public APIs should not swallow arbitrary invalid keyword arguments

[glyph]

This needs to be done in three steps:

• [x] emit a deprecation warning when emitting unused keyword arguments
• [x] do a release with the deprecation warning
• [x] remove the **kwargs entirely

251 was both the initial report of this as well as a proposed implementation, but its author lost interest.

[twm]

https://github.com/twisted/treq/pull/297 adds deprecation warnings. It needs review.

[twm]

https://github.com/twisted/treq/pull/319 removes **kwargs entirely.