Modernize code to Python 3.6+ (#1158)

* Compact dict and set building
* Remove redundant parentheses
* Simplify chained conditions
* Change method name to lowercase
* Use triple double quotes for docstrings

Thanks, @eumiro!

datasette/renderer.py

@@ -27,7 +27,7 @@ def convert_specific_columns_to_json(rows, columns, json_cols):
 
 
 def json_renderer(args, data, view_name):
-    """ Render a response as JSON """
+    """Render a response as JSON"""
     status_code = 200
     # Handle the _json= parameter which may modify data["rows"]
     json_cols = []