stdout is not restored when printing a traceback

[pawamoy]

Describe the bug
If an exception is raised in line-by-line mode, the standard output descriptor is not restored, so the JSON error is not printed, and it makes mkdocstrings hang since there's no output to read.

Expected behavior
stdout should be properly restored when an exception is caught in line-by-line mode.

Additional context
We should move the code responsible for discarding/restoring stdout into the main function of the cli module, so we can restore it in the except handler, and in the finally clause.