Implement None == None

Fixes #111

tests/snippets/json_snippet.py

@@ -38,7 +38,7 @@ assert "str" == json.loads('"str"')
 assert True == json.loads('true')
 assert False == json.loads('false')
 # TODO: uncomment once None comparison is implemented
-# assert None == json.loads('null')
+assert None == json.loads('null')
 assert [] == json.loads('[]')
 assert ['a'] == json.loads('["a"]')
 assert [['a'], 'b'] == json.loads('[["a"], "b"]')

vm/src/pyobject.rs

@@ -779,6 +779,7 @@ impl PartialEq for PyObject {
                 }
             }
             (PyObjectKind::Boolean { value: a }, PyObjectKind::Boolean { value: b }) => a == b,
+            (PyObjectKind::None, PyObjectKind::None) => true,
             _ => panic!(
                 "TypeError in COMPARE_OP: can't compare {:?} with {:?}",
                 self, other