pythongh-110864: TypeVar constructor: Partially revert pythongh-110784, constraints cannot be NULL (python#110922)

Author: sobolevn

Objects/typevarobject.c

@@ -363,26 +363,20 @@ typevar_new_impl(PyTypeObject *type, PyObject *name, PyObject *constraints,
         }
     }
 
-    if (constraints != NULL) {
-        if (!PyTuple_CheckExact(constraints)) {
-            PyErr_SetString(PyExc_TypeError,
-                            "constraints must be a tuple");
-            return NULL;
-        }
-        Py_ssize_t n_constraints = PyTuple_GET_SIZE(constraints);
-        if (n_constraints == 1) {
-            PyErr_SetString(PyExc_TypeError,
-                            "A single constraint is not allowed");
-            Py_XDECREF(bound);
-            return NULL;
-        } else if (n_constraints == 0) {
-            constraints = NULL;
-        } else if (bound != NULL) {
-            PyErr_SetString(PyExc_TypeError,
-                            "Constraints cannot be combined with bound=...");
-            Py_XDECREF(bound);
-            return NULL;
-        }
+    assert(PyTuple_CheckExact(constraints));
+    Py_ssize_t n_constraints = PyTuple_GET_SIZE(constraints);
+    if (n_constraints == 1) {
+        PyErr_SetString(PyExc_TypeError,
+                        "A single constraint is not allowed");
+        Py_XDECREF(bound);
+        return NULL;
+    } else if (n_constraints == 0) {
+        constraints = NULL;
+    } else if (bound != NULL) {
+        PyErr_SetString(PyExc_TypeError,
+                        "Constraints cannot be combined with bound=...");
+        Py_XDECREF(bound);
+        return NULL;
     }
     PyObject *module = caller();
     if (module == NULL) {