Add python linting step to travis build

.style.yapf

@@ -1,11 +0,0 @@
-[style]
-coalesce_brackets = true
-column_limit=99
-dedent_closing_brackets = true
-each_dict_entry_on_separate_line = true
-indent_dictionary_value = true
-join_multiple_lines = false
-split_arguments_when_comma_terminated = true
-split_before_first_argument = true
-split_before_logical_operator = true
-split_before_named_assigns = true

.travis.yml

@@ -13,4 +13,10 @@ before_script:
 script:
   - pytest --cov=misago
 after_success:
-  - coveralls
+  - coveralls
+matrix:
+  include:
+    - python: 3.6
+      script:
+        - pip install black
+        - black --check devproject misago