bpo-8901: Windows registry path is now ignored with the -E option (GH-18169)

Doc/whatsnew/3.9.rst

@@ -548,6 +548,11 @@ Build and C API Changes
 * The :c:func:`PyModule_AddType` function is added to help adding a type to a module.
   (Contributed by Dong-hee Na in :issue:`40024`.)
 
+* The Windows registry is no longer used to initialize :data:`sys.path` when
+  the ``-E`` option is used. This is significant when embedding Python on
+  Windows.
+  (Contributed by Zackery Spytz in :issue:`8901`.)
+
 Deprecated
 ==========
 

Misc/NEWS.d/next/Windows/2020-01-24-09-15-41.bpo-8901.hVnhGO.rst

@@ -0,0 +1 @@
+Ignore the Windows registry when the ``-E`` option is used.

PC/getpathp.c

@@ -783,8 +783,11 @@ calculate_module_search_path(PyCalculatePath *calculate,
 {
     int skiphome = calculate->home==NULL ? 0 : 1;
 #ifdef Py_ENABLE_SHARED
-    calculate->machine_path = getpythonregpath(HKEY_LOCAL_MACHINE, skiphome);
+    if (!Py_IgnoreEnvironmentFlag) {
+        calculate->machine_path = getpythonregpath(HKEY_LOCAL_MACHINE,
+                                                   skiphome);
         calculate->user_path = getpythonregpath(HKEY_CURRENT_USER, skiphome);
+    }
 #endif
     /* We only use the default relative PYTHONPATH if we haven't
        anything better to use! */