actions: Add muliprocessing start-method to matrix

[zmedico]

Add multiprocessing start-method to matrix so that we are prepared for when python changes to default to spawn. Exclude all python versions except 3.12-dev for now.

This adds a conditional ci step that patches the sources just for the spawn start-method. It patches all bin/* scripts with python shebangs, and also patches the test command in meson.build.

Bug: https://bugs.gentoo.org/914876

@thesamesam this does something like what you suggested in #1249 (comment). This implementation is kind of ugly, but maybe good enough for the first iteration. I've also added this to #1251.

[thesamesam]

It's gnarly but disappointingly, there's no way to control it from the command line (was really hoping for something like the new var for CPU cores). Thank you!

[zmedico]

I temporarily added a multiprocessing.get_start_method() == "fork" to one of the tests, and inadvertently found out that somehow multiprocessing.set_start_method("spawn", force=True) in the pytest command is practically ignored when the pytest-xdist plugin is used. We still get a lot of useful coverage anyway from the patched bin/* python scripts, but it will be nice if we can fix the pytest-xdist plugin somehow. Seems related to execnet usage, which has support form threading models but seemingly not multiprocessing models.

Maybe we just need to patch the portage.tests.TestCase setUp method to call multiprocessing.set_start_method("spawn", force=True). Yeah, this seems to work:

--- a/lib/portage/tests/__init__.py
+++ b/lib/portage/tests/__init__.py
@@ -1,3 +1,3 @@
 # tests/__init__.py -- Portage Unit Test functionality
-# Copyright 2006-2023 Gentoo Authors
+# Copyright 2006-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
@@ -5,2 +5,3 @@
 import argparse
+import multiprocessing
 import sys
@@ -81,2 +82,6 @@ class TestCase(unittest.TestCase):
 
+    def setUp(self):
+        if os.environ.get("PORTAGE_MULTIPROCESSING_START_METHOD") == "spawn":
+            multiprocessing.set_start_method("spawn", force=True)
+
     def assertRaisesMsg(self, msg, excClass, callableObj, *args, **kwargs):

Sent #1265.