Use type annotations on class attributes

This is a workaround for PyCQA/flake8#1755,
but also a shift towards the bright Python 3 future.

tests.py

@@ -14,7 +14,7 @@
 from contextlib import closing
 from functools import partial
 from io import BytesIO, StringIO
-from typing import Optional
+from typing import Dict, Optional
 from xml.etree import ElementTree as ET
 
 
@@ -932,8 +932,8 @@ def test_zest_releaser_check_failure_user_plods_on(self, check_manifest,
 class VCSHelper:
 
     # override in subclasses
-    command = None  # type: Optional[str]
-    extra_env = {}
+    command: Optional[str] = None
+    extra_env: Dict[str, str] = {}
 
     @property
     def version(self):