Sort keys in configtypes.Dict.to_str

Otherwise, we get inconsistent output between runs on Python 3.5

qutebrowser/config/configtypes.py

@@ -1295,7 +1295,7 @@ class Dict(BaseType):
         if not value:
             # An empty Dict is treated just like None -> empty string
             return ''
-        return json.dumps(value)
+        return json.dumps(value, sort_keys=True)
 
     def to_doc(self, value, indent=0):
         if not value:

tests/unit/completion/test_models.py

@@ -844,8 +844,8 @@ def test_setting_option_completion(qtmodeltester, config_stub,
         "Options": [
             ('aliases', 'Aliases for commands.', '{"q": "quit"}'),
             ('bindings.commands', 'Default keybindings', (
-                '{"normal": {"<Ctrl+q>": "quit", "ZQ": "quit", '
-                '"I": "invalid", "d": "scroll down"}}')),
+                '{"normal": {"<Ctrl+q>": "quit", "I": "invalid", '
+                '"ZQ": "quit", "d": "scroll down"}}')),
             ('completion.open_categories', 'Which categories to show (in '
              'which order) in the :open completion.',
              '["searchengines", "quickmarks", "bookmarks", "history"]'),

tests/unit/config/test_configtypes.py

@@ -1703,6 +1703,11 @@ class TestDict:
         value = typ.from_obj({'1': '2'})
         assert value == {'1': 2}
 
+    def test_to_str(self, klass):
+        typ = klass(keytype=configtypes.String(), valtype=configtypes.String())
+        d = {'a': 'b', 'c': 'd'}
+        assert typ.to_str(d) == '{"a": "b", "c": "d"}'
+
 
 def unrequired_class(**kwargs):
     return configtypes.File(required=False, **kwargs)