Python: flush stdout and stderr after running Python script

When debugging with gdb in vscode, the stuff I print when executing a script in
the text editor does not show up in the terminal. It does work when I flush
explicitly though using `print(..., flush=True)`. This is quite annoying.

The solution is to always flush `stdout` and `stderr` automatically when running
a script. This is done using the CPython API, as just using
`fflush(stdout/stderr)` did not solve the issue.

Pull Request: https://projects.blender.org/blender/blender/pulls/136632

source/blender/python/intern/bpy_interface_run.cc

@@ -217,6 +217,10 @@ static bool python_script_exec(
 
   PyC_MainModule_Restore(main_mod);
 
+  /* Flush stdout/stderr to ensure the script output is visible.
+   * Using fflush(stdout) does not solve it. */
+  PyC_StdFilesFlush();
+
   bpy_context_clear(C, &gilstate);
 
   return (py_result != nullptr);