Started out as progress towards #1476, but I ended up fixing a few more things:
- fixed the signature of _encode_type, which actually returns a pair, not a string
- made some attributes into properties in order to prevent the descriptor protocol from turning them into methods
- found a bug in CPython in the process (python/cpython#6779)
I used the following test file to make sure these classes are now instantiable:
```python
import codecs
import io
from typing import IO
bio = io.BytesIO()
cod = codecs.lookup('utf-8')
codecs.StreamReaderWriter(bio, codecs.StreamReader, codecs.StreamWriter)
codecs.StreamRecoder(bio, cod.encode, cod.decode, codecs.StreamReader, codecs.StreamWriter)
```
