sockeserver: Add undocumented internals (#3924)

the `rfile` and `wfile` members are already implemented by
StreamRequestHandler. In addition to them several (undocumented)
class and instance variables exist according to
<https://github.com/python/cpython/blob/master/Lib/socketserver.py#L742>:
- `rbufsize`
- `wbufsize`
- `timeout`
- `disable_nagle_algorithm`
- `packet` and `socket` for datagrams

The already exist with Python 2.7
<https://github.com/python/cpython/blob/2.7/Lib/SocketServer.py#L677>

```mermaid
classDiagram
BaseRequestHandler <|-- DatagramRequestHandler
BaseRequestHandler <|-- StreamRequestHandler
StreamRequestHandler <|-- BaseHTTPRequestHandler
```

stdlib/3/http/server.pyi

@@ -27,8 +27,6 @@ class BaseHTTPRequestHandler(socketserver.StreamRequestHandler):
     path: str
     request_version: str
     headers: email.message.Message
-    rfile: BinaryIO
-    wfile: BinaryIO
     server_version: str
     sys_version: str
     error_message_format: str