new test for 'is None' in Python bool expression

tests/run/isnonebool.pyx

+__doc__ = """
+    >>> test_and(None, None)
+    True
+    >>> test_and(None, 1)
+    False
+    >>> test_and(1, None)
+    False
+
+    >>> test_more(None, None)
+    True
+    >>> test_more(None, 1)
+    True
+    >>> test_more(1, None)
+    False
+    >>> test_more(None, 0)
+    False
+"""
+
+def test_and(a,b):
+    return a is None and b is None
+
+def test_more(a,b):
+    return a is None and (b is None or b == 1)