Ensure the set object description is reproducible.

Whilst working on the Reproducible Builds effort [0], we noticed
that sphinx could generate output that is not reproducible.

In particular, the rendering of `set` objects in default arguments
and elsewhere is currently non-determinstic. For example:

    class A_Class(object):
        a_set = {'a', 'b', 'c'}

Might be rendered as any of:

     {'a', 'b', 'c'}
     {'a', 'c', 'b'}
     {'b', 'a', 'c'}
     {'b', 'c', 'a'}
     {'c', 'a', 'b'}
     {'c', 'b', 'a'}

Patch attached that sorts the contents of sets whilst rendering.
This is parallel to the `dict` key sorting.

This was originally filed in Debian as #895553 [1].

 [0] https://reproducible-builds.org/
 [1] https://bugs.debian.org/895553

Signed-off-by: Chris Lamb <lamby@debian.org>

tests/test_util_inspect.py

@@ -346,6 +346,24 @@ def test_dictionary_sorting():
     assert description == "{'a': 1, 'b': 4, 'c': 3, 'd': 2}"
 
 
+def test_set_sorting():
+    set_ = set("gfedcba")
+    description = inspect.object_description(set_)
+    if PY3:
+        assert description == "{'a', 'b', 'c', 'd', 'e', 'f', 'g'}"
+    else:
+        assert description == "set(['a', 'b', 'c', 'd', 'e', 'f', 'g'])"
+
+
+def test_set_sorting_fallback():
+    set_ = set((None, 1))
+    description = inspect.object_description(set_)
+    if PY3:
+        assert description in ("{1, None}", "{None, 1}")
+    else:
+        assert description in ("set([1, None])", "set([None, 1])")
+
+
 def test_dict_customtype():
     class CustomType(object):
         def __init__(self, value):