Fix typo in ref_output_0 variable in test_python.
Tested by running the selftest on x86_64-linux with python 3.11.
SELF_CHECK (saw_exception);
std::string ref_output_0 ("Traceback (most recent call last):\n"
" File \"<string>\", line 0, in <module>\n"
- " KeyboardInterrupt\n");
+ "KeyboardInterrupt\n");
std::string ref_output_1 ("Traceback (most recent call last):\n"
" File \"<string>\", line 1, in <module>\n"
"KeyboardInterrupt\n");