Rules: indentation: Handle sets

Sets are like mappings, that do not contain values. Example:

    set:
      ? key one
      ? key two
      ? [non, scalar, key]

tests/test_spec_examples.py

@@ -110,6 +110,7 @@ conf_overrides = {
     'example-8.9': ('empty-lines: {max-end: 1}\n'),
     'example-8.14': ('colons: {max-spaces-before: 1}\n'),
     'example-8.16': ('indentation: {spaces: 1}\n'),
+    'example-8.17': ('indentation: disable\n'),
 }
 
 files = os.listdir('tests/yaml-1.2-spec-examples')
@@ -125,9 +126,7 @@ def _gen_test(buffer, conf):
 # The following tests are blacklisted because they contain rarely-used formats
 # that yamllint does not handle yet.
 tmp_blacklist = (
-     'example-2.25',
      'example-7.16',
-     'example-8.17',
      'example-8.20',
      'example-8.22',
      'example-10.1',

tests/test_syntax_errors.py

@@ -75,3 +75,20 @@ class YamlLintTestCase(RuleTestCase):
                    '   Atlanta Braves]\n'
                    ': [2001-07-02, 2001-08-12,\n'
                    '   2001-08-14]\n', None)
+
+    def test_sets(self):
+        self.check('---\n'
+                   '? key one\n'
+                   '? key two\n'
+                   '? [non, scalar, key]\n'
+                   '? key with value\n'
+                   ': value\n'
+                   '...\n', None)
+        self.check('---\n'
+                   '? - multi\n'
+                   '  - line\n'
+                   '  - keys\n'
+                   '? in:\n'
+                   '    a:\n'
+                   '      set\n'
+                   '...\n', None)