sets: correct Interval.boundary to satisfy the standard topology of ℝ

diofant/sets/sets.py

@@ -448,8 +448,7 @@ def is_open(self):
         boundary
 
         """
-        if not Intersection(self, self.boundary):
-            return True
+        return not Intersection(self, self.boundary)
 
     @property
     def is_closed(self):
@@ -949,7 +948,9 @@ def _union(self, other):
 
     @property
     def boundary(self):
-        return FiniteSet(self.start, self.end)
+        return FiniteSet(*(p for p, c in [(self.start, not self.left_open),
+                                          (self.end, not self.right_open)]
+                           if abs(p) != oo or c))
 
     def _contains(self, other):
         if not isinstance(other, Expr) or other in (nan, zoo):

diofant/tests/sets/test_fancysets.py

@@ -228,8 +228,11 @@ def test_reals():
     assert sqrt(-1) not in S.Reals
     assert S.Reals == Interval.open(-oo, oo)
     assert hash(S.Reals) == hash(Interval.open(-oo, oo))
+    assert S.Reals.is_open is True
+    assert S.Reals.is_closed is True
     assert S.ExtendedReals == Interval(-oo, oo)
     assert hash(S.ExtendedReals) == hash(Interval(-oo, oo))
+    assert S.ExtendedReals.is_closed is True
 
 
 def test_intersections():