gh-110864: TypeVar constructor: Partially revert gh-110784, constraints cannot be NULL (#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) {