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().

Objects/dictobject.c

@@ -3853,14 +3853,22 @@ static PyObject *
 dictview_repr(_PyDictViewObject *dv)
 {
     PyObject *seq;
-    PyObject *result;
+    PyObject *result = NULL;
+    Py_ssize_t rc;
 
+    rc = Py_ReprEnter((PyObject *)dv);
+    if (rc != 0) {
+        return rc > 0 ? PyUnicode_FromString("...") : NULL;
+    }
     seq = PySequence_List((PyObject *)dv);
-    if (seq == NULL)
-        return NULL;
-
+    if (seq == NULL) {
+        goto Done;
+    }
     result = PyUnicode_FromFormat("%s(%R)", Py_TYPE(dv)->tp_name, seq);
     Py_DECREF(seq);
+
+Done:
+    Py_ReprLeave((PyObject *)dv);
     return result;
 }