bpo-33012: Fix invalid function cast warnings with gcc 8 in Argument Clinic. (GH-6748)

Fix invalid function cast warnings with gcc 8
for method conventions different from METH_NOARGS, METH_O and
METH_VARARGS in Argument Clinic generated code.
diff --git a/Objects/clinic/floatobject.c.h b/Objects/clinic/floatobject.c.h
index ac3ff0c..8ff2a2b 100644
--- a/Objects/clinic/floatobject.c.h
+++ b/Objects/clinic/floatobject.c.h
@@ -47,7 +47,7 @@
 "When an argument is passed, work like built-in round(x, ndigits).");
 
 #define FLOAT___ROUND___METHODDEF    \
-    {"__round__", (PyCFunction)float___round__, METH_FASTCALL, float___round____doc__},
+    {"__round__", (PyCFunction)(void(*)(void))float___round__, METH_FASTCALL, float___round____doc__},
 
 static PyObject *
 float___round___impl(PyObject *self, PyObject *o_ndigits);
@@ -256,7 +256,7 @@
 "This affects how floats are converted to and from binary strings.");
 
 #define FLOAT___SET_FORMAT___METHODDEF    \
-    {"__set_format__", (PyCFunction)float___set_format__, METH_FASTCALL|METH_CLASS, float___set_format____doc__},
+    {"__set_format__", (PyCFunction)(void(*)(void))float___set_format__, METH_FASTCALL|METH_CLASS, float___set_format____doc__},
 
 static PyObject *
 float___set_format___impl(PyTypeObject *type, const char *typestr,
@@ -305,4 +305,4 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=a3c366a156be61f9 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=091dd499f5386a6c input=a9049054013a1b77]*/