bpo-31333: Re-implement ABCMeta in C (GH-5733)

This adds C versions of methods used by ABCMeta that
improve performance of various ABC operations.
diff --git a/PC/config.c b/PC/config.c
index 0c08bfe..568a0fa 100644
--- a/PC/config.c
+++ b/PC/config.c
@@ -5,6 +5,7 @@
 
 #include "Python.h"
 
+extern PyObject* PyInit__abc(void);
 extern PyObject* PyInit_array(void);
 extern PyObject* PyInit_audioop(void);
 extern PyObject* PyInit_binascii(void);
@@ -80,6 +81,7 @@
 
 struct _inittab _PyImport_Inittab[] = {
 
+    {"_abc", PyInit__abc},
     {"array", PyInit_array},
     {"_ast", PyInit__ast},
     {"audioop", PyInit_audioop},