Correcting test, when math.tau was added since Python 3.6.

Author: ppisl

graalpython/com.oracle.graal.python.test/src/tests/test_math.py

@@ -106,7 +106,9 @@ def testConstants(self):
         # Ref: Abramowitz & Stegun (Dover, 1965)
         self.ftest('pi', math.pi, 3.141592653589793238462643)
         self.ftest('e', math.e, 2.718281828459045235360287)
-        self.assertEqual(math.tau, 2*math.pi)
+        if (sys.version_info.major >= 3 and sys.version_info.minor >= 6):
+            # math.tau since 3.6
+            self.assertEqual(math.tau, 2*math.pi)
 
     def test_ceil_basic(self):
         self.assertEqual(math.ceil(10), 10)