pyembed: BUG: SIGSEGV in safe code

#344 adds a test for the bug that this issue tracks: a segmentation fault coming from safe code:

```rust
let py = {
let mut config = pyembed::OxidizedPythonInterpreterConfig::default();
config.interpreter_config.parse_argv = Some(false);
config.set_missing_path_configuration = false;
let mut interp = pyembed::MainPythonInterpreter::new(config).unwrap();
interp.acquire_gil().unwrap()
};
py.import("sys").unwrap();
```

I would expect lifetime bounds to prevent this from compiling.

Start with the regression test referenced in issue #344 and trace the lifetime relationships around MainPythonInterpreter, acquire_gil(), and py.import("sys"). The fix is complete when the shown safe-code example no longer compiles because of lifetime bounds, and the regression test confirms the segmentation fault cannot occur.

Written by the indexing model from the issue text.