text: Title: `--snapshot-default-extension` doesn't support pytest 7 `pythonpath`
  Body: **Describe the bug**
  
  `--snapshot-default-extension` doesn't support pytest 7's `pythonpath` configuration option, for pytest-only additions to the Python path.
  
  For my project, I'm using `--snapshot-default-extension` so the right extension and serializer are in place, before Syrupy begins its reporting. My Syrupy extensions are for tests only, so they live outside of my src/ folder. Only the src/ folder of my project seems to be on the default Python path. So when running tests, I need to tell Syrupy about my extensions, somehow. I'd love to use the vanilla `pytest` command directly, configured in pyproject.toml, without having to pass a custom `PYTHONPATH` to `pytest` every time.
  
  **To reproduce**
  
  See my branch, [john-kurkowski/syrupy#default-extension-pythonpath](https://github.com/john-kurkowski/syrupy/compare/main..default-extension-pythonpath). In the final commit,  https://github.com/john-kurkowski/syrupy/commit/ea9779371583253c03b0bdf47c09ca6f5526d909, switching from modifying `sys.path` to setting pytest's `--pythonpath` breaks 2/3 of the branch's test cases. **EDIT:** pytest's `pythonpath` an INI configuration option, not CLI.
  
  ```diff
  diff --git a/tests/integration/test_snapshot_option_extension.py b/tests/integration/test_snapshot_option_extension.py
  index de8e807..42b2eec 100644
  --- a/tests/integration/test_snapshot_option_extension.py
  +++ b/tests/integration/test_snapshot_option_extension.py
  @@ -26,11 +26,11 @@ def testfile(testdir):
       return testdir
   
   
  -def test_snapshot_default_extension_option_success(monkeypatch, testfile):
  -    monkeypatch.syspath_prepend(testfile.tmpdir)
  -
  +def test_snapshot_default_extension_option_success(testfile):
       result = testfile.runpytest(
           "-v",
  +        "--pythonpath",
  +        testfile.tmpdir,
           "--snapshot-update",
           "--snapshot-default-extension",
           "extension_file.MySingleFileExtension",
  @@ -63,11 +63,11 @@ def test_snapshot_default_extension_option_module_not_found(testfile):
       assert result.ret
   
   
  -def test_snapshot_default_extension_option_member_not_found(monkeypatch, testfile):
  -    monkeypatch.syspath_prepend(testfile.tmpdir)
  -
  +def test_snapshot_default_extension_option_member_not_found(testfile):
       result = testfile.runpytest(
           "-v",
  +        "--pythonpath",
  +        testfile.tmpdir,
           "--snapshot-update",
           "--snapshot-default-extension",
           "extension_file.DoesNotExistExtension",
  ```
  
  **Expected behavior**
  
  Tests in my branch should pass.
  
  **Environment:**
  
  - OS: macOS
  - Syrupy Version: 4.0.1
  - Python Version: 3.11.1
  
  **Workaround**
  
  Set `PYTHONPATH` prior to invoking the pytest CLI. 
  
  ```sh
  PYTHONPATH=path/to/my/extensions/folder pytest --snapshot-default-extension some_module.SomeExtension
  ```
  
  