Add typing support

[dave-shawley]

I started an effort to do this a few years ago (see #23) but it fizzled since the typing SIG was still changing things. Now that things have settled done in the part of the ecosystem, I'd like to reboot it. I still like the approach that I took in my original branch:

• represent expectations using structural sub-typing and typing.Protocol sub-classes explicitly
• add type definitions for precisely we know how to serialize -- IOW, define a union type like SerializableTypes
• document the definitions and expected usage
• pull mypy (maybe pyright?) into the build chain

The one downside is that typing.Protocol was added in 3.8 and we still support 3.7. We can use the typing.extensions package as a backport. For this reason, I would isolate the typing-related classes into a module named sprockets.mixins.mediatype.type_info and use it in conjunction with typing. The goal is to not let the non-3.7 compatibility stuff leak out of the module.

Feel free to steal, adopt, and/or ignore portions of my original branch.