Handle the case with type hint in the pre-3.6 style

Also handle notmal comment not starting with 'type:' better.

src/black/__init__.py

@@ -2710,7 +2710,11 @@ def make_comment(content: str) -> str:
     if content[0] == "#":
         content = content[1:]
     NON_BREAKING_SPACE = " "
-    if content and content[0] == NON_BREAKING_SPACE and "type: ignore" not in content:
+    if (
+        content
+        and content[0] == NON_BREAKING_SPACE
+        and not content.lstrip().startswith("type:")
+    ):
         content = " " + content[1:]  # Replace NBSP by a simple space
     if content and content[0] not in " !:#'%":
         content = " " + content

tests/data/comments7.py

@@ -129,6 +129,8 @@ def test_fails_invalid_post_data(
     ):
         ...
 
+square = Square(4) # type: Optional[Square]
+
 # output
 
 from .config import (
@@ -263,4 +265,7 @@ class C:
     def test_fails_invalid_post_data(
         self, pyramid_config, db_request, post_data, message
     ):
-        ...
+        ...
+
+
+square = Square(4)  # type: Optional[Square]