Take "ownership" of "our" version of the IRI class

The IRI class was originally externally maintained and imported into the project, however the external project has since ceased to exist and the class has had very little love over the years.

Along the same lines as the use in Requests, the IRI class is also still in use in the SimplePie project and possibly in other projects.

I propose we take ownership of the IRI class as used in Requests properly.

To do this, I see the following tasks:

• Find the top 5 other projects which still use the IRI class.
• Compare their current version with our version and see what we can learn from this.
• Update the Requests IRI class with best practices from the other uses, including updating/adding tests (and giving credit where credit is due!!!).
• Update the class docblock to fix the original copyright (name change) and add additional copyright for the newly brought in changes, including changes made by the Requests team.
• Remove the "exception" status from the IRI class, i.e. make it comply with the Requests coding standards and remove any other exceptions in CI/QA for the IRI class.

Once these tasks are done, the IRI class should, as of that point in time, be treated like any other class in Requests and no longer have the "externally maintained" exception status.