setup.py fails to recognize repository checkouts

### Description

`python/setup.py:is_git_repo()` checks `Path(__file__).parent / ".git"`. Because `setup.py` lives under `python/`, this tests `python/.git` rather than the repository root.

At current main commit `8260bc34398c2b8f36dc840fd22f741ca9294584`, the check returns `False` in a normal checkout even though the root `.git` directory exists.

### Reproduction

Evaluating the relevant setup helpers from the fixed checkout gives:

```text
is_git_repo() = False
get_git_version_suffix() = ''
git rev-parse --short=8 HEAD = 8260bc34
```

### Impact

- development builds omit the intended `+git` version suffix
- `BackendInstaller.prepare()` skips its source-checkout submodule preparation branch

### Expected behavior

The helper should recognize the repository root one directory above `python/`. The check should also accept a `.git` file so linked worktrees are recognized.

Start in python/setup.py with is_git_repo() and get_git_version_suffix(), then inspect how BackendInstaller.prepare() uses the checkout check. Reproduce the reported helper results in a normal checkout, including the git rev-parse command. Done means the repository root one directory above python/ is recognized for both .git directories and files, so the version suffix and checkout preparation branch are no longer skipped.

Written by the indexing model from the issue text.