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
@@ -4617,7 +4621,7 @@ def TErr(err_msg: str) -> Err[CannotTransform]:
 def contains_pragma_comment(comment_list: List[Leaf]) -> bool:
     """
     Returns:
-        True iff one of the comments in @comment_list is a pragma used by one
+        True if one of the comments in @comment_list is a pragma used by one
         of the more common static analysis tools for python (e.g. mypy, flake8,
         pylint).
     """

tests/data/comments7.py

@@ -129,6 +129,8 @@ def test_fails_invalid_post_data(
     ):
         ...
 
+square = Square(4) # type: Optional[Square]
+
 # output
 
 from .config import (
@@ -199,7 +201,7 @@ def func():
         a[-1],  # type: ignore
     )
 
-    #  The type: ignore exception only applies to line length, not
+    # The type: ignore exception only applies to line length, not
     # other types of formatting.
     c = call(
         "aaaaaaaa",
@@ -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]