Partially revert "Allow Sage to work with a system Python 3.6."

This reverts commit abb5607.

src/sage/combinat/subset.py

@@ -353,25 +353,20 @@ def cardinality(self):
             8
             sage: Subsets(3).cardinality()
             8
-        """
-        return Integer(1) << self._s.cardinality()
-
-    def __len__(self):
-        r"""
-        Equivalent to ``self.cardinality()``.
 
         TESTS::
 
-            ``__len__`` should return a Python int; in Python 3.7+ this happens
-            automatically, but not on Python 3.6.
+            ``__len__`` should return a Python int.
 
             sage: S = Subsets(Set([1,2,3]))
             sage: len(S)
             8
             sage: type(len(S)) is int
             True
         """
-        return int(self.cardinality())
+        return Integer(1) << self._s.cardinality()
+
+    __len__ = cardinality
 
     def first(self):
         """

src/sage/graphs/views.pyx

@@ -626,7 +626,6 @@ cdef class EdgesView:
         elif i < 0:
             return list(self)[i]
         else:
-            i = int(i)  # For Python < 3.7 where islice doesn't support non-int
             try:
                 return next(islice(self, i, i + 1, 1))
             except StopIteration: