typing: Add more tests for TypeVar (#104571)

During the PEP 695 implementation at one point I made
TypeVar.__name__ return garbage, and all of test_typing passed.
So I decided to add a few more tests. In the process I discovered
a minor incompatibility from the C implementation of TypeVar:
empty constraints were returned as None instead of an empty tuple.

Objects/typevarobject.c

@@ -271,7 +271,7 @@ typevar_constraints(typevarobject *self, void *Py_UNUSED(ignored))
         return Py_NewRef(self->constraints);
     }
     if (self->evaluate_constraints == NULL) {
-        Py_RETURN_NONE;
+        return PyTuple_New(0);
     }
     PyObject *constraints = PyObject_CallNoArgs(self->evaluate_constraints);
     self->constraints = Py_XNewRef(constraints);