Update exchange API to be capsule following new convention

This PR updates the exchange API to be capsule following the new convention.
Note that the exchange API python recommendation is not yet updated on the array-api
side and so far there hasn't yet been downstream implementation, so this change
should be safe in generally.

Author: tqchen

include/dlpack/dlpack.h

@@ -19,7 +19,7 @@
 #define DLPACK_MAJOR_VERSION 1
 
 /*! \brief The current minor version of dlpack */
-#define DLPACK_MINOR_VERSION 2
+#define DLPACK_MINOR_VERSION 3
 
 /*! \brief DLPACK_DLL prefix for windows */
 #ifdef _WIN32
@@ -375,7 +375,7 @@ typedef struct DLManagedTensorVersioned {
 } DLManagedTensorVersioned;
 
 //----------------------------------------------------------------------
-// DLPack `__c_dlpack_exchange_api__` fast exchange protocol definitions
+// DLPack `__dlpack_c_exchange_api__` fast exchange protocol definitions
 //----------------------------------------------------------------------
 /*!
  * \brief Request a producer library to create a new tensor.
@@ -531,16 +531,17 @@ typedef struct DLPackExchangeAPIHeader {
  * \brief Framework-specific function pointers table for DLPack exchange.
  *
  * Additionally to `__dlpack__()` we define a C function table sharable by
- * Python implementations via `__c_dlpack_exchange_api__`.
- * This attribute must be set on the type as a Python integer compatible
- * with `PyLong_FromVoidPtr`/`PyLong_AsVoidPtr`.
+ *
+ * Python implementations via `__dlpack_c_exchange_api__`.
+ * This attribute must be set on the type as a Python PyCapsule
+ * with name "dlpack_exchange_api".
  *
  * A consumer library may use a pattern such as:
  *
  * \code
  *
- * PyObject *api_obj = type(tensor_obj).__c_dlpack_exchange_api__;  // as C-code
- * MyDLPackExchangeAPI *api = PyLong_AsVoidPtr(api_obj);
+ * PyObject *api_obj = type(tensor_obj).__dlpack_c_exchange_api__;  // as C-code
+ * MyDLPackExchangeAPI *api = PyCapsule_GetPointer(api_obj, "dlpack_exchange_api");
  * if (api == NULL && PyErr_Occurred()) { goto handle_error; }
  *
  * \endcode