subscription-manager cli command does not support IPv6 proxy

Description of problem:
Hosts in IPv6-only networks cannot subscribe to RHSM using an IPv6 proxy address. The dual-stack proxy is necessary as RHSM currently still don't support IPv6.

Version-Release number of selected component (if applicable):
subscription-manager 1.29.33.1-1.el9_2

How reproducible:
100%

Steps to Reproduce:
1. Provision host with IPv6 connectivity
2. Run `subscription-manager version --proxy="http://[<IPv6-of-your-proxy>]:3128

Actual results:
The following blocks show you some examples of the the commands I tried. IP addresses were replaced with RFC 3849 prefix.

~~~

1. subscription-manager version --proxy="http://[2001:DB8::DEAD:BEEF:2]:3128"
   Traceback (most recent call last):
   File "/usr/sbin/subscription-manager", line 33, in <module>
   sys.exit(load_entry_point('subscription-manager==1.29.33.1', 'console_scripts', 'subscription-manager')())
   File "/usr/lib64/python3.9/site-packages/subscription_manager/scripts/subscription_manager.py", line 73, in main
   return managercli.ManagerCLI().main()
   File "/usr/lib64/python3.9/site-packages/subscription_manager/managercli.py", line 87, in main
   ret = CLI.main(self)
   File "/usr/lib64/python3.9/site-packages/subscription_manager/cli.py", line 190, in main
   return cmd.main()
   File "/usr/lib64/python3.9/site-packages/subscription_manager/cli_command/cli.py", line 331, in main
   self.proxy_port = int(parts[1])
   ValueError: invalid literal for int() with base 10: 'DB8'
   ~~~

The same 'ValueError' occurs when specifying the IPv6 address without the brackets '[]'. Using proxy FQDN leads to the same exception followed by another one:

~~~

1. subscription-manager version --proxy="http://proxy.example.com"
   Traceback (most recent call last):
   File "/usr/lib64/python3.9/site-packages/rhsm/utils.py", line 207, in parse_url
   int(port)
   ValueError: invalid literal for int() with base 10: 'DB8'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/sbin/subscription-manager", line 33, in <module>
sys.exit(load_entry_point('subscription-manager==1.29.33.1', 'console_scripts', 'subscription-manager')())
File "/usr/lib64/python3.9/site-packages/subscription_manager/scripts/subscription_manager.py", line 73, in main
return managercli.ManagerCLI().main()
File "/usr/lib64/python3.9/site-packages/subscription_manager/managercli.py", line 87, in main
ret = CLI.main(self)
File "/usr/lib64/python3.9/site-packages/subscription_manager/cli.py", line 190, in main
return cmd.main()
File "/usr/lib64/python3.9/site-packages/subscription_manager/cli_command/cli.py", line 376, in main
self.cp = self.cp_provider.get_consumer_auth_cp()
File "/usr/lib64/python3.9/site-packages/subscription_manager/cp_provider.py", line 184, in get_consumer_auth_cp
self.consumer_auth_cp = connection.UEPConnection(
File "/usr/lib64/python3.9/site-packages/rhsm/connection.py", line 1367, in _init_
super(UEPConnection, self)._init_(user_agent=user_agent, **kwargs)
File "/usr/lib64/python3.9/site-packages/rhsm/connection.py", line 248, in _init_
info = utils.get_env_proxy_info()
File "/usr/lib64/python3.9/site-packages/rhsm/utils.py", line 237, in get_env_proxy_info
info = parse_url(proxy_info, default_port=DEFAULT_PROXY_PORT)
File "/usr/lib64/python3.9/site-packages/rhsm/utils.py", line 211, in parse_url
raise ServerUrlParseErrorPort(local_server_entry)
rhsm.utils.ServerUrlParseErrorPort: http://[2001:DB8::DEAD:BEEF:2]:3128/
~~~

Expected results:
Command should succeed.