Add event loop setup for Python 3.14 compatibility

[malfet]

Set a new event loop for asyncio in Python 3.14 and above.

Otherwise invoking ghstack crashes with

ERROR: Fatal exception
Traceback (most recent call last):
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/logs.py", line 105, in manager
    yield
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/cli.py", line 47, in cli_context
    yield shell, config, github
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/cli.py", line 281, in submit
    ghstack.submit.main(
    ~~~~~~~~~~~~~~~~~~~^
        msg=message,
        ^^^^^^^^^^^^
    ...<13 lines>...
        direct_opt=direct_opt,
        ^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/submit.py", line 259, in main
    submitter = Submitter(**kwargs)
  File "<string>", line 24, in __init__
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/submit.py", line 379, in __post_init__
    repo = ghstack.github_utils.get_github_repo_info(
        github=self.github,
    ...<4 lines>...
        remote_name=self.remote_name,
    )
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/github_utils.py", line 71, in get_github_repo_info
    name_with_owner = get_github_repo_name_with_owner(
        sh=sh,
        github_url=github_url,
        remote_name=remote_name,
    )
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/github_utils.py", line 28, in get_github_repo_name_with_owner
    remote_url = sh.git("remote", "get-url", remote_name)
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/shell.py", line 281, in git
    return self._maybe_rstrip(self.sh(*(("git",) + args), **kwargs))
                              ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nshulga/py3.14-local/lib/python3.14/site-packages/ghstack/shell.py", line 200, in sh
    loop = asyncio.get_event_loop()
  File "/opt/homebrew/Cellar/[email protected]/3.14.0_1/Frameworks/Python.framework/Versions/3.14/lib/python3.14/asyncio/events.py", line 715, in get_event_loop
    raise RuntimeError('There is no current event loop in thread %r.'
                       % threading.current_thread().name)
RuntimeError: There is no current event loop in thread 'MainThread'.

[ezyang]

Put it in main instead?