Issue #13959: Re-implement imp.get_suffixes() in Lib/imp.py.

This introduces a new function, imp.extension_suffixes(), which is
currently undocumented. That is forthcoming once issue #14657 is
resolved and how to expose file suffixes is decided.
diff --git a/Python/dynload_next.c b/Python/dynload_next.c
index 5caff8b..85c95b4 100644
--- a/Python/dynload_next.c
+++ b/Python/dynload_next.c
@@ -8,10 +8,7 @@
 
 #include <mach-o/dyld.h>
 
-const struct filedescr _PyImport_DynLoadFiletab[] = {
-    {".so", "rb", C_EXTENSION},
-    {0, 0}
-};
+const char *_PyImport_DynLoadFiletab[] = {".so", NULL};
 
 /*
 ** Python modules are Mach-O MH_BUNDLE files. The best way to load these