[ci] Fix approx in autodiff example test

Fixing broken master.

 #3782 didn't catch this since it was on an older master base
commit.

tests/python/examples/autodiff/test_minimization.py

@@ -17,4 +17,4 @@ def test_minimization():
         gradient_descent()
 
     for i in range(n):
-        assert x[i] == ti.approx(y[i], rel=1e-3)
+        assert x[i] == ti._testing.approx(y[i], rel=1e-3)