lint-all: Check for use of '== None'.

tools/lint-all

@@ -155,6 +155,9 @@ python_rules = [
     {'pattern': "^\s+[#]\w",
      'strip': '\n',
      'description': 'Missing whitespace after "#"'},
+    {'pattern': "== None",
+     'exclude': 'tools/lint-all',
+     'description': 'Use `is None` to check whether something is None'},
     {'pattern': ", [)]",
      'description': 'Unnecessary whitespace between "," and ")"'},
     {'pattern': "%  [(]",

zerver/lib/validator.py

@@ -20,7 +20,7 @@ for arbitrarily complex validators.  See ValidatorTestCase for example usage.
 
 A simple example of composition is this:
 
-   check_list(check_string)('my_list', ['a', 'b', 'c']) == None
+   check_list(check_string)('my_list', ['a', 'b', 'c']) is None
 
 To extend this concept, it's simply a matter of writing your own validator
 for any particular type of object.