Fix py3 socket slots

[carsonip]

Was casually reading gevent source code, found something interesting: python3 socket is not using slots properly. _read_event and _write_event instance attrs are added to __dict__ instead of __slots__. The problematic class attrs mask the slots, and that change was made in 2018 and the SocketMixin with slots was added in 2020. I think it should be just a careless mistake (and it didn't do much harm indeed).

Before change (gevent master, py38):

In [1]: from gevent import socket                                                                                                                     

In [2]: s = socket.socket()                                                                                                                           

In [3]: s.__dict__                                                                                                                                    
Out[3]: 
{'timeout': None,
 'hub': <Hub '' at 0x7f1c51043ac0 epoll default pending=0 ref=0 fileno=18 thread_ident=0x7f1c53d64740>,
 '_read_event': <io at 0x7f1c509d1740 native=0x7f1c509d1780 fd=17 events=READ|_IOFDSET>,
 '_write_event': <io at 0x7f1c509d1840 native=0x7f1c509d1880 fd=17 events=WRITE|_IOFDSET>}

In [12]: %timeit -r 12 s._read_event                                                                                                                  
36.1 ns ± 0.272 ns per loop (mean ± std. dev. of 12 runs, 10000000 loops each)

In [17]: sys.getsizeof(s.__dict__)                                                                                                                    
Out[17]: 104

After change:

In [1]: from gevent import socket                                                                                                                     

In [2]: s = socket.socket()                                                                                                                           

In [3]: s.__dict__                                                                                                                                    
Out[3]: 
{'timeout': None,
 'hub': <Hub '' at 0x7f587b6a1f40 epoll default pending=0 ref=0 fileno=18 thread_ident=0x7f587e4db740>}

In [4]: s._read_event                                                                                                                                 
Out[4]: <io at 0x7f587b7dcb40 native=0x7f587b7dcb80 fd=17 events=READ|_IOFDSET>                                                                                                                               

In [6]: sys.getsizeof(s.__dict__)                                                                                                                     
Out[6]: 104

In [15]: %timeit -r 12 s._read_event                                                                                                                  
33.3 ns ± 0.173 ns per loop (mean ± std. dev. of 12 runs, 10000000 loops each)

No memory usage change. Marginal performance gain (<10%) from using slots for attr access. My laptop produces unstable microbenchmarks so you may get a different number.

[carsonip]

Or maybe we can once and for all ditch the instance dict, add timeout and hub to the mixin slots and add empty slots to socket2 and socket3. But there are self.__dict__['timeout'] below that makes it not so straightforward.

[jamadden]

That's a good idea, thanks.