Enterprise Teams support

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.

Start in auth.py at _get_installation_id, where the traceback shows the installation account is matched by login. Compare the enterprise account fields described in the issue and verify installation-token generation for an enterprise installation; done means the lookup no longer raises KeyError and selects the intended installation.