[Bugfix] Fix ipv6 address parsing bug

[liiliiliil]

In a pure IPv6 environment, the get_distributed_init_method function in vllm/utils.py returns a URL such as "tcp://2001:4860:4860::8888:8086", where "2001:4860:4860::8888" is the IP address. This format leads to an error when urllib.parse.urlparse attempts to parse the URL and extract the port number:

>>> urllib.parse.urlparse("tcp://2001:4860:4860::8888:8086").port
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/root/mambaforge/envs/vllm/lib/python3.10/urllib/parse.py", line 185, in port
    raise ValueError(f"Port could not be cast to integer value as {port!r}")
ValueError: Port could not be cast to integer value as '4860:4860::8888:8086'

We can resolve this issue by constructing the URL with square brackets around the IPv6 address, like so: "tcp://[2001:4860:4860::8888]:8086". With this format, the port is correctly parsed:

>>> urllib.parse.urlparse("tcp://[2001:4860:4860::8888]:8086").port
8086

This fix addresses a new bug encountered following the fix for issue #2685

[njhill]

@liiliiliil @simon-mo this seems to have broken the non-ipv6 case, please check #3645 🙏

[liiliiliil]

@njhill The ipv4 environment works for me with python 3.10. Didn't know it is a bug report here python/cpython#103848. Thank you for the additional fix. :)

>>> urllib.parse.urlparse("tcp://[127.0.0.1]:8086").port
8086