bpo-18533: Avoid RecursionError from repr() of recursive dictview (#4823)

dictview_repr(): Use a Py_ReprEnter() / Py_ReprLeave() pair to check
for recursion, and produce "..." if so.

test_recursive_repr(): Check for the string rather than a
RecursionError.  (Test cannot be any tighter as contents are
implementation-dependent.)

test_deeply_nested_repr(): Add new test, replacing the original
test_recursive_repr().  It checks that a RecursionError is raised in
the case of a non-recursive but deeply nested structure.  (Very
similar to what test_repr_deep() in test/test_dict.py does for a
normal dict.)

OrderedDictTests: Add new test case, to test behavior on OrderedDict
instances containing their own values() or items().

Lib/test/test_dictviews.py

@@ -1,6 +1,7 @@
 import collections.abc
 import copy
 import pickle
+import sys
 import unittest
 
 class DictSetTest(unittest.TestCase):
@@ -202,6 +203,20 @@ class DictSetTest(unittest.TestCase):
     def test_recursive_repr(self):
         d = {}
         d[42] = d.values()
+        r = repr(d)
+        # Cannot perform a stronger test, as the contents of the repr
+        # are implementation-dependent.  All we can say is that we
+        # want a str result, not an exception of any sort.
+        self.assertIsInstance(r, str)
+        d[42] = d.items()
+        r = repr(d)
+        # Again.
+        self.assertIsInstance(r, str)
+
+    def test_deeply_nested_repr(self):
+        d = {}
+        for i in range(sys.getrecursionlimit() + 100):
+            d = {42: d.values()}
         self.assertRaises(RecursionError, repr, d)
 
     def test_copy(self):