tests/test_cli.py | 47 41 + 6 - 0 !
1 file changed, 41 insertions(+), 6 deletions(-)

 fix a test failure caused by an internal docutils `open()` call

docutils 0.22 introduced an RST stylesheet feature
that must be able to open files during RST-to-HTML rendering.

The original design of the `test_cli_explicit_format` test
patched `pathlib.Path.open()` so it would always return the
same open file instance...which was always closed on first use.
This caused a failure when docutils tried to open `minimal.css`:

```
ValueError: I/O operation on closed file.
```

This change introduces a more complex mocking strategy
that documents and meets the current technical requirements.