tests: test_graph imports the names it uses

Replaces the star import, which Codacy reports as F405 on every changed line
that uses one of the names.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: petercorke

tests/test_graph.py

@@ -3,7 +3,7 @@
 import numpy as np
 import numpy.testing as nt
 
-from pgraph import *
+from pgraph import DGraph, Edge, UGraph, Vertex
 
 class TestUGraph(unittest.TestCase):