bpo-40453: Add PyConfig._isolated_subinterpreter (GH-19820)

An isolated subinterpreter cannot spawn threads, spawn a child
process or call os.fork().

* Add private _Py_NewInterpreter(isolated_subinterpreter) function.
* Add isolated=True keyword-only parameter to
  _xxsubinterpreters.create().
* Allow again os.fork() in "non-isolated" subinterpreters.
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 0bdfae1..3d60b2f 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -406,6 +406,7 @@
         'check_hash_pycs_mode': 'default',
         'pathconfig_warnings': 1,
         '_init_main': 1,
+        '_isolated_interpreter': 0,
     }
     if MS_WINDOWS:
         CONFIG_COMPAT.update({
@@ -766,6 +767,8 @@
 
             'check_hash_pycs_mode': 'always',
             'pathconfig_warnings': 0,
+
+            '_isolated_interpreter': 1,
         }
         self.check_all_configs("test_init_from_config", config, preconfig,
                                api=API_COMPAT)