bpo-32604: Make _xxsubinterpreters build on Windows (GH-5516)

This is not the ideal solution; this means that a test module is now
always included in the main python3x.dll.  However, we're already
including xxsubtype, so why not?

PC/config.c

@@ -31,7 +31,9 @@ extern PyObject* PyInit__locale(void);
 #endif
 extern PyObject* PyInit__codecs(void);
 extern PyObject* PyInit__weakref(void);
+/* XXX: These two should really be extracted to standalone extensions. */
 extern PyObject* PyInit_xxsubtype(void);
+extern PyObject* PyInit__xxsubinterpreters(void);
 extern PyObject* PyInit_zipimport(void);
 extern PyObject* PyInit__random(void);
 extern PyObject* PyInit_itertools(void);
@@ -126,6 +128,7 @@ struct _inittab _PyImport_Inittab[] = {
     {"_json", PyInit__json},
 
     {"xxsubtype", PyInit_xxsubtype},
+    {"_xxsubinterpreters", PyInit__xxsubinterpreters},
     {"zipimport", PyInit_zipimport},
 #ifdef _Py_HAVE_ZLIB
     {"zlib", PyInit_zlib},

PCbuild/pythoncore.vcxproj

@@ -279,6 +279,7 @@
     <ClCompile Include="..\Modules\_tracemalloc.c" />
     <ClCompile Include="..\Modules\timemodule.c" />
     <ClCompile Include="..\Modules\xxsubtype.c" />
+    <ClCompile Include="..\Modules\_xxsubinterpretersmodule.c" />
     <ClCompile Include="..\Modules\zipimport.c" />
     <ClCompile Include="..\Modules\_io\fileio.c" />
     <ClCompile Include="..\Modules\_io\bytesio.c" />