Optimise internal fallback method calls in Py3.7+ by directly unpacking the method calls.

Closes #2850.

Cython/Utility/ObjectHandling.c

@@ -1807,6 +1807,14 @@ static PyObject* __Pyx_PyObject_CallMethod0(PyObject* obj, PyObject* method_name
     PyObject *method = NULL, *result = NULL;
     int is_method = __Pyx_PyObject_GetMethod(obj, method_name, &method);
     if (likely(is_method)) {
+#if PY_VERSION_HEX >= 0x030700A0 && CYTHON_UNPACK_METHODS && CYTHON_FAST_PYCCALL
+        if (Py_TYPE(method) == &PyMethodDescr_Type) {
+            PyMethodDescrObject *descr = (PyMethodDescrObject *)method;
+            result = _PyMethodDef_RawFastCallKeywords(descr->d_method, obj, NULL, 0, NULL);
+            if (unlikely(!result))
+                result = _Py_CheckFunctionResult(obj, result, NULL);
+        } else
+#endif
         result = __Pyx_PyObject_CallOneArg(method, obj);
         Py_DECREF(method);
         return result;
@@ -1839,6 +1847,14 @@ static PyObject* __Pyx_PyObject_CallMethod1(PyObject* obj, PyObject* method_name
     PyObject *method = NULL, *result;
     int is_method = __Pyx_PyObject_GetMethod(obj, method_name, &method);
     if (likely(is_method)) {
+#if PY_VERSION_HEX >= 0x030700A1 && CYTHON_UNPACK_METHODS && CYTHON_FAST_PYCCALL
+        if (Py_TYPE(method) == &PyMethodDescr_Type) {
+            PyMethodDescrObject *descr = (PyMethodDescrObject *)method;
+            result = _PyMethodDef_RawFastCallKeywords(descr->d_method, obj, &arg, 1, NULL);
+            if (unlikely(!result))
+                result = _Py_CheckFunctionResult(obj, result, NULL);
+        } else
+#endif
         result = __Pyx_PyObject_Call2Args(method, obj, arg);
         Py_DECREF(method);
         return result;
@@ -1890,6 +1906,15 @@ static PyObject* __Pyx_PyObject_CallMethod2(PyObject* obj, PyObject* method_name
     PyObject *args, *method = NULL, *result = NULL;
     int is_method = __Pyx_PyObject_GetMethod(obj, method_name, &method);
     if (likely(is_method)) {
+#if PY_VERSION_HEX >= 0x030700A0 && CYTHON_UNPACK_METHODS && CYTHON_FAST_PYCCALL
+        if (Py_TYPE(method) == &PyMethodDescr_Type) {
+            PyMethodDescrObject *descr = (PyMethodDescrObject *)method;
+            PyObject *args[2] = {arg1, arg2};
+            result = _PyMethodDef_RawFastCallKeywords(descr->d_method, obj, args, 2, NULL);
+            if (unlikely(!result))
+                result = _Py_CheckFunctionResult(obj, result, NULL);
+        } else
+#endif
         result = __Pyx_PyObject_Call3Args(method, obj, arg1, arg2);
         Py_DECREF(method);
         return result;