bpo-43688: Support the limited C API in debug mode (GH-25131)

The limited C API is now supported if Python is built in debug mode
(if the Py_DEBUG macro is defined). In the limited C API, the
Py_INCREF() and Py_DECREF() functions are now implemented as opaque
function calls, rather than accessing directly the PyObject.ob_refcnt
member, if Python is built in debug mode and the Py_LIMITED_API macro
targets Python 3.10 or newer. It became possible to support the
limited C API in debug mode because the PyObject structure is the
same in release and debug mode since Python 3.8 (see bpo-36465).

The limited C API is still not supported in the --with-trace-refs
special build (Py_TRACE_REFS macro).
diff --git a/setup.py b/setup.py
index 75bd163..3b4e7ae 100644
--- a/setup.py
+++ b/setup.py
@@ -1864,17 +1864,11 @@ def detect_modules(self):
 ##         # Uncomment these lines if you want to play with xxmodule.c
 ##         self.add(Extension('xx', ['xxmodule.c']))
 
-        if 'd' not in sysconfig.get_config_var('ABIFLAGS'):
-            # Non-debug mode: Build xxlimited with limited API
-            self.add(Extension('xxlimited', ['xxlimited.c'],
-                               define_macros=[('Py_LIMITED_API', '0x030a0000')]))
-            self.add(Extension('xxlimited_35', ['xxlimited_35.c'],
-                               define_macros=[('Py_LIMITED_API', '0x03050000')]))
-        else:
-            # Debug mode: Build xxlimited with the full API
-            # (which is compatible with the limited one)
-            self.add(Extension('xxlimited', ['xxlimited.c']))
-            self.add(Extension('xxlimited_35', ['xxlimited_35.c']))
+        # Limited C API
+        self.add(Extension('xxlimited', ['xxlimited.c'],
+                           define_macros=[('Py_LIMITED_API', '0x030a0000')]))
+        self.add(Extension('xxlimited_35', ['xxlimited_35.c'],
+                           define_macros=[('Py_LIMITED_API', '0x03050000')]))
 
     def detect_tkinter_fromenv(self):
         # Build _tkinter using the Tcl/Tk locations specified by