Add the u-sigil for unicode (as they did), add the b-sigil for bytestrings (as they eventually did), and then go through a regular deprecation cycle for sigil-l

Add the u-sigil for unicode (as they did), add the b-sigil for bytestrings (as they eventually did), and then go through a regular deprecation cycle for sigil-less strings (rather than releasing a 3.0 where the u-sigils were removed completely). Maybe at some point re-add sigil-less strings as an alias for u-strings, but I'd rather have old stuff break with a clear message than have a bunch of weird side bugs. Do the same for the type names.