allow disabling the ABC registration of the Cython Generator and Coroutine...

allow disabling the ABC registration of the Cython Generator and Coroutine types to support compiling the "abc" module in CPython's stdlib itself

Cython/Utility/Coroutine.c

@@ -1585,6 +1585,10 @@ static int __Pyx_patch_abc(void); /*proto*/
 //////////////////// PatchGeneratorABC ////////////////////
 //@requires: PatchModuleWithCoroutine
 
+#ifndef CYTHON_REGISTER_ABCS
+#define CYTHON_REGISTER_ABCS 1
+#endif
+
 #if defined(__Pyx_Generator_USED) || defined(__Pyx_Coroutine_USED)
 static PyObject* __Pyx_patch_abc_module(PyObject *module); /*proto*/
 static PyObject* __Pyx_patch_abc_module(PyObject *module) {
@@ -1607,7 +1611,7 @@ if _cython_coroutine_type is not None:
 static int __Pyx_patch_abc(void) {
 #if defined(__Pyx_Generator_USED) || defined(__Pyx_Coroutine_USED)
     static int abc_patched = 0;
-    if (!abc_patched) {
+    if (CYTHON_REGISTER_ABCS && !abc_patched) {
         PyObject *module;
         module = PyImport_ImportModule((PY_VERSION_HEX >= 0x03030000) ? "collections.abc" : "collections");
         if (!module) {