Fix casts in C implementation for GCC 14

Fix casts performed in the C extension module to make it build with
GCC 14.  This involves adding two missing casts from `Blake3Object *`
to `PyObject *` for return values, and fixing casts in method
definitions to use `PyCFunction` (i.e. the type expected by the struct)
rather than `PyCFunctionWithKeywords` (i.e. the type that's actually
used by the function, and that wouldn't require a cast in the first
place).

Fixes oconnor663#43

c_impl/blake3module.c

@@ -217,7 +217,7 @@ static PyObject *Blake3_update(Blake3Object *self, PyObject *args) {
   // Success. We need to increment the refcount on self to return it, see:
   // https://docs.python.org/3/extending/extending.html#ownership-rules.
   Py_INCREF(self);
-  ret = self;
+  ret = (PyObject *)self;
 
 exit:
   release_buf_if_acquired(&data);
@@ -272,7 +272,7 @@ static PyObject *Blake3_update_mmap(Blake3Object *self, PyObject *args,
   // Success. We need to increment the refcount on self to return it, see:
   // https://docs.python.org/3/extending/extending.html#ownership-rules.
   Py_INCREF(self);
-  ret = self;
+  ret = (PyObject *)self;
 
 exit:
   if (file) {
@@ -347,11 +347,11 @@ static PyObject *Blake3_reset(Blake3Object *self, PyObject *args) {
 
 static PyMethodDef Blake3_methods[] = {
     {"update", (PyCFunction)Blake3_update, METH_VARARGS, "add input bytes"},
-    {"update_mmap", (PyCFunctionWithKeywords)Blake3_update_mmap,
+    {"update_mmap", (PyCFunction)Blake3_update_mmap,
      METH_VARARGS | METH_KEYWORDS, "add input bytes from a filepath"},
-    {"digest", (PyCFunctionWithKeywords)Blake3_digest,
+    {"digest", (PyCFunction)Blake3_digest,
      METH_VARARGS | METH_KEYWORDS, "finalize the hash"},
-    {"hexdigest", (PyCFunctionWithKeywords)Blake3_hexdigest,
+    {"hexdigest", (PyCFunction)Blake3_hexdigest,
      METH_VARARGS | METH_KEYWORDS,
      "finalize the hash and encode the result as hex"},
     {"copy", (PyCFunction)Blake3_copy, METH_VARARGS,