This PR tracks the new Converter System, which is currently the biggest blocker for v0.8 (see #245).
The first commit takes the first big step towards a more ergonomic (and esp. extensible) converter hierarchy, sketching how the initialization around the contextmanager-local, non-threadsafe ConverterContext and its Converter(Factory) mapping works. A few things that are now nicer than before

• Converters can store their data locally as fields instead of using the unwieldy context dict together with obfuscated keys
• the extra context param has been dropped everywhere, the few places that explicitly need it get it from a thread local variable
• all context data together with the current converters and their configuration are now collected within ConverterContext, which should make managing this easier (less weird keys in dicts flying around)
• things are still thread-safe (or even more so) as different threads have different Converter(-Context) instances
• this even allows for different configurations with different converters (e.g. compliance to different RFCs / versions) in the same process
• customization should be a lot easier as fewer things are immutable
• initialization is no longer based on module imports, so this should also be easier to handle and extend
• I removed some complexity around ImmutableComponentMeta and __attrs_post_init__ for immutable classes
• the whole systems now makes a lot more sense in my head and should be easier to explain (while still being somewhat complicated, but I guess some of that is inherent)
• ValueConverters are now singletons and thus easier to handle
• ...probably more things which I forgot

The specific Converters still need to be patched to use the new context to store data. So there's still further clean-up to do before this even compiles, and then this needs to be tested, and I also want to include some basic documentation on what I did and how extensions could work. Maybe I even have time to polish some further things like removing the intermediate attrs classes now that we can customize __init__ and the RuntimeAttrValidation that is now built-in to attrs. I'll update this PR once I got to all (or at least some) of that.

This leaves error-handling / variable strictness and recurring events on the ToDo list, but I guess we can just as well defer these features for 0.9 as long as we get these pretty deep-rooted changes here through before anybody strongly relies on how the converters work.

PS: sorry for opening this so shortly before the "blackening", I really wanted to get these changes finally visible. Still, I guess this shouldn't block the mass-reformat, as I can probably "rebase" my changes applying black to every/the single commit here.