Interactive client requires Python>=3.9.0

[paskozdilar]

Running python3 -m websockets wss://echo.websocket.org/ on Python 3.8.5 gives an uninformative error:

Traceback (most recent call last):                                                                                                                                                                                 
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main                                                                                                                                             
    return _run_code(code, main_globals, None,                                                                                                                                                                     
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code                                                                                                                                                        
    exec(code, run_globals)                                                                                                                                                                                        
  File "/home/$USER/websockets/src/websockets/__main__.py", line 215, in <module>                                                                                                                            
    main()                                                                                                                                                                                                         
  File "/home/$USER/websockets/src/websockets/__main__.py", line 181, in main                                                                                                                                
    async def queue_factory() -> asyncio.Queue[str]:                                                                                                                                                               
TypeError: 'type' object is not subscriptable

The issue is related to the PEP 585 - Type Hinting Generics In Standard Collections, which is introduced in Python 3.9.

This does not fit the Python version requirement specified in the documentation.

The fix could be either:

• quoting the asyncio.Queue[str] on the line 181 of __main__.py, or
• importing from __future__ import annotations the backwards compatility.

NOTE: If the latter fix is chosen, I think it would be style-consistent to convert other quoted type hints (on lines 94 and 95) to type-hinted generics.

I have submitted a PR with the latter fix.

[aaugustin]

from __future__ import annotations doesn't work on Python 3.6, which is supported in 9.x (and removed in 10.x) so we have to go with quoting.