Enterprise support

[shtrom]

If using an app which has been installed at an enterprise level, there is a failure is auth.py looking for the account login.

  File "/home/shtrom/work/reviewer-selector/src/reviewer_selector/github.py", line 54, in async_generate_github_token
    token = await inst_auth.get_token()
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 200, in get_token
    return await anext(self._generator)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 13, in anext
    return await ait.__anext__()
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 169, in _gen_installation_token
    installation_id = await self._get_installation_id()
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 146, in _get_installation_id
    if installation["account"]["login"] == self.owner:
       ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
KeyError: 'login'

This is because enterprises have an account.slug rather than an account.owner.

[shtrom]

The problem seems to run deeper, as enterprises get completely separate installation IDs which only seem to have enterprise permissions, and none of the other.

Two issues so far:

1. the owner of a repo is not an enterprise, so the enterprise installation ID will never be selected (this would however be sufficient to avoid failing if an enterprise account exists)
2. when forcibly selecting the enterprise through dark magic leads to a 500 from GitHub, maybe because we're not supposed to get Enterprise token?

> /home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py(147)_get_installation_id()
-> if installation["account"].get("login") == self.owner:
(Pdb) self.owner='enterprise'
(Pdb) c
Traceback (most recent call last):
  File "/home/shtrom/work/reviewer-selector/.venv/bin/reviewer-selector", line 10, in <module>
    sys.exit(cli())
             ^^^^^
  File "/home/shtrom/work/reviewer-selector/src/reviewer_selector/cli.py", line 64, in cli
    reviewable.add_reviewers(resolved)
  File "/home/shtrom/work/reviewer-selector/src/reviewer_selector/github.py", line 163, in add_reviewers
    self._pr.authenticated_api_request(
  File "/home/shtrom/work/reviewer-selector/src/reviewer_selector/github.py", line 101, in wrapped
    self._gh_token = self._gh_app.generate_token()
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/src/reviewer_selector/github.py", line 39, in generate_token
    return asyncio.run(
           ^^^^^^^^^^^^
  File "/home/shtrom/.local/share/uv/python/cpython-3.12.11-linux-x86_64-gnu/lib/python3.12/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/home/shtrom/.local/share/uv/python/cpython-3.12.11-linux-x86_64-gnu/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/.local/share/uv/python/cpython-3.12.11-linux-x86_64-gnu/lib/python3.12/asyncio/base_events.py", line 691, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/src/reviewer_selector/github.py", line 54, in async_generate_github_token
    token = await inst_auth.get_token()
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 203, in get_token
    return await anext(self._generator)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 13, in anext
    return await ait.__anext__()
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 186, in _gen_installation_token
    token = await _gentoken()
            ^^^^^^^^^^^^^^^^^
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/simple_github/auth.py", line 181, in _gentoken
    response.raise_for_status()
  File "/home/shtrom/work/reviewer-selector/.venv/lib/python3.12/site-packages/aiohttp/client_reqrep.py", line 655, in raise_for_status
    raise ClientResponseError(
aiohttp.client_exceptions.ClientResponseError: 500, message='Internal Server Error', url='https://api.github.com/app/installations/xxx/access_tokens'