pylint: Enable new extensions

See https://pylint.pycqa.org/en/latest/whatsnew/2/2.16/index.html#extensions

.pylintrc

@@ -16,6 +16,8 @@ load-plugins=qute_pylint.config,
              pylint.extensions.typing,
              pylint.extensions.docparams,
              pylint.extensions.private_import,
+             pylint.extensions.dict_init_mutate,
+             pylint.extensions.dunder
 
 persistent=n
 py-version=3.7

tests/unit/browser/webkit/test_mhtml.py

@@ -94,8 +94,8 @@ def test_refuses_non_ascii_header_value(checker, header, value):
         'root_content': b'',
         'content_location': 'http://example.com',
         'content_type': 'text/plain',
+        header: value,
     }
-    defaults[header] = value
     writer = mhtml.MHTMLWriter(**defaults)
     with pytest.raises(UnicodeEncodeError, match="'ascii' codec can't encode"):
         writer.write_to(checker.fp)