Rework socket (#5545)

* Extract _socket.pyi from socket.pyi.
* Extract _socket.socket from socket.socket.
* Fix socket.family annotation.
* Annotate SocketIO properly.
* SocketType is an alias of _socket.socket.
* Sort items in socket.pyi in the same order as in socket.py.
* Remove socket.EINTR.
* Use _typeshed.WriteableBuffer instead of custom alias.
* Add errorTab (Windows only).
* Add _socket.dup().
* Mark positional-only argments.
* Remove constructors from socket exceptions.
* socket.timeout is an alias for TimeoutError, starting with Python 3.10.
* Use PEP 604 in changed lines.
* Add alias for fileno arguments.
* getaddrinfo() port can be bytes.
* Explicitly override some SSLSocket methods.
* Allow ReadableBuffer in _CMSG arguments.

stdlib/VERSIONS

@@ -43,6 +43,7 @@ _py_abc: 3.7-
 _pydecimal: 3.6-
 _random: 2.7-
 _sitebuiltins: 3.6-
+_socket: 3.0-  # present in 2.7 at runtime, but not in typeshed
 _stat: 3.6-
 _thread: 2.7-
 _threading_local: 3.6-