improve codecs stubs (#2114)

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)
```

Author: JelleZijlstra

stdlib/2and3/codecs.pyi

@@ -2,6 +2,7 @@
 # https://docs.python.org/2/library/codecs.html and https://docs.python.org/3/library/codecs.html
 import sys
 from typing import (
+    Any,
     BinaryIO,
     Callable,
     Generator,
@@ -33,8 +34,8 @@ _encoded = bytes
 
 # TODO: It is not possible to specify these signatures correctly, because
 # they have an optional positional or keyword argument for errors=.
-_encode_type = Callable[[_decoded], _encoded]  # signature of Codec().encode
-_decode_type = Callable[[_encoded], _decoded]  # signature of Codec().decode
+_encode_type = Callable[[_decoded], Tuple[_encoded, int]]  # signature of Codec().encode
+_decode_type = Callable[[_encoded], Tuple[_decoded, int]]  # signature of Codec().decode
 _stream_reader_type = Callable[[IO[_encoded]], 'StreamReader']  # signature of StreamReader __init__
 _stream_writer_type = Callable[[IO[_encoded]], 'StreamWriter']  # signature of StreamWriter __init__
 _incremental_encoder_type = Callable[[], 'IncrementalEncoder']  # signature of IncrementalEncoder __init__
@@ -49,13 +50,19 @@ def decode(obj: _encoded, encoding: str = ..., errors: str = ...) -> _decoded:
 def lookup(encoding: str) -> 'CodecInfo':
     ...
 class CodecInfo(Tuple[_encode_type, _decode_type, _stream_reader_type, _stream_writer_type]):
-    encode = ...  # type: _encode_type
-    decode = ...  # type: _decode_type
-    streamreader = ...  # type: _stream_reader_type
-    streamwriter = ...  # type: _stream_writer_type
-    incrementalencoder = ...  # type: _incremental_encoder_type
-    incrementaldecoder = ...  # type: _incremental_decoder_type
-    name = ...  # type: str
+    @property
+    def encode(self) -> _encode_type: ...
+    @property
+    def decode(self) -> _decode_type: ...
+    @property
+    def streamreader(self) -> _stream_reader_type: ...
+    @property
+    def streamwriter(self) -> _stream_writer_type: ...
+    @property
+    def incrementalencoder(self) -> _incremental_encoder_type: ...
+    @property
+    def incrementaldecoder(self) -> _incremental_decoder_type: ...
+    name: str
     def __init__(self, encode: _encode_type, decode: _decode_type, streamreader: _stream_reader_type = ..., streamwriter: _stream_writer_type = ..., incrementalencoder: _incremental_encoder_type = ..., incrementaldecoder: _incremental_decoder_type = ..., name: str = ...) -> None: ...
 
 def getencoder(encoding: str) -> _encode_type:
@@ -207,7 +214,10 @@ class StreamReaderWriter(TextIO):
     def read(self, size: int= ...) -> _decoded: ...
     def readline(self, size: Optional[int] = ...) -> _decoded: ...
     def readlines(self, sizehint: Optional[int] = ...) -> List[_decoded]: ...
-    def __next__(self) -> _decoded: ...
+    if sys.version_info >= (3,):
+        def __next__(self) -> Text: ...
+    else:
+        def next(self) -> Text: ...
     def __iter__(self: _T) -> _T: ...
     # This actually returns None, but that's incompatible with the supertype
     def write(self, data: _decoded) -> int: ...
@@ -217,7 +227,49 @@ class StreamReaderWriter(TextIO):
     def seek(self, offset: int, whence: int = ...) -> int: ...
     def __enter__(self: _T) -> _T: ...
     def __exit__(self, typ: Optional[Type[BaseException]], exc: Optional[BaseException], tb: Optional[types.TracebackType]) -> bool: ...
+    def __getattr__(self, name: str) -> Any: ...
+
+    # These methods don't actually exist directly, but they are needed to satisfy the TextIO
+    # interface. At runtime, they are delegated through __getattr__.
+    def close(self) -> None: ...
+    def fileno(self) -> int: ...
+    def flush(self) -> None: ...
+    def isatty(self) -> bool: ...
+    def readable(self) -> bool: ...
+    def truncate(self, size: Optional[int] = ...) -> int: ...
+    def seekable(self) -> bool: ...
+    def tell(self) -> int: ...
+    def writable(self) -> bool: ...
+
+_SRT = TypeVar('_SRT', bound=StreamRecoder)
 
 class StreamRecoder(BinaryIO):
     def __init__(self, stream: IO[_encoded], encode: _encode_type, decode: _decode_type, Reader: _stream_reader_type, Writer: _stream_writer_type, errors: str = ...) -> None:
         ...
+    def read(self, size: int = ...) -> bytes: ...
+    def readline(self, size: Optional[int] = ...) -> bytes: ...
+    def readlines(self, sizehint: Optional[int] = ...) -> List[bytes]: ...
+    if sys.version_info >= (3,):
+        def __next__(self) -> bytes: ...
+    else:
+        def next(self) -> bytes: ...
+    def __iter__(self: _SRT) -> _SRT: ...
+    def write(self, data: bytes) -> int: ...
+    def writelines(self, list: Iterable[bytes]) -> int: ...  # type: ignore  # it's supposed to return None
+    def reset(self) -> None: ...
+    def __getattr__(self, name: str) -> Any: ...
+    def __enter__(self: _SRT) -> _SRT: ...
+    def __exit__(self, type: Optional[Type[BaseException]], value: Optional[BaseException], tb: Optional[types.TracebackType]) -> bool: ...
+
+    # These methods don't actually exist directly, but they are needed to satisfy the BinaryIO
+    # interface. At runtime, they are delegated through __getattr__.
+    def seek(self, offset: int, whence: int = ...) -> int: ...
+    def close(self) -> None: ...
+    def fileno(self) -> int: ...
+    def flush(self) -> None: ...
+    def isatty(self) -> bool: ...
+    def readable(self) -> bool: ...
+    def truncate(self, size: Optional[int] = ...) -> int: ...
+    def seekable(self) -> bool: ...
+    def tell(self) -> int: ...
+    def writable(self) -> bool: ...