Support multiprocessing in spawn mode

[abdnh]

This adds support for spawning Python processes.

See beeware/briefcase-windows-VisualStudio-template#128 for context.

Depends on beeware/Python-Apple-support#369

Closes #20

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Claude Opus 5.5

[freakboy3742]

Needs a guard here to avoid setting when subprocesses are not allowed:

Suggested change

	// Point sys.executable at bin/python3 so that the app can start Python subprocesses.
	path = [python_home stringByAppendingPathComponent:@"bin/python3"];
	if ([[NSFileManager defaultManager] isExecutableFileAtPath:path]) {
	debug_log(@"Setting sys.executable: %@", path);
	module_attr = PyUnicode_FromString([path UTF8String]);
	if (module_attr == NULL
	|| PySys_SetObject("executable", module_attr) < 0
	|| PySys_SetObject("_base_executable", module_attr) < 0) {
	crash_dialog(@"Could not set sys.executable");
	exit(-16);
	}
	Py_DECREF(module_attr);
	}
	{% if cookiecutter.allow_subprocesses -%}
	// Point sys.executable at bin/python3 so that the app can start Python subprocesses.
	path = [python_home stringByAppendingPathComponent:@"bin/python3"];
	if ([[NSFileManager defaultManager] isExecutableFileAtPath:path]) {
	debug_log(@"Setting sys.executable: %@", path);
	module_attr = PyUnicode_FromString([path UTF8String]);
	if (module_attr == NULL
	|| PySys_SetObject("executable", module_attr) < 0
	|| PySys_SetObject("_base_executable", module_attr) < 0) {
	crash_dialog(@"Could not set sys.executable");
	exit(-16);
	}
	Py_DECREF(module_attr);
	}
	{% endif -%}

[freakboy3742]

Wait - I see what you've done here - you're checking at runtime - nice catch.

An alternate approach: Given that calling a subprocess is explosive (especially on macOS, but generally as well), it might be a good idea to set sys.executable to None (or a similar "unusable" value) if the subprocesses aren't allowed.