Do not treat "... # comment" as a dummy implementation

Fixes psf#4063

CHANGES.md

@@ -16,6 +16,7 @@
 
 - Additional cases of immediately nested tuples, lists, and dictionaries are now
   indented less (#4012)
+- Keep suites consisting of an ellipsis followed by a comment on their own lines (#4066)
 
 ### Configuration
 

src/black/nodes.py

@@ -765,6 +765,10 @@ def is_stub_body(node: LN) -> bool:
     if len(node.children) != 2:
         return False
 
+    # If there is a comment after the ..., don't treat this suite as a stub body.
+    if node.children[1].prefix.strip():
+        return False
+
     child = node.children[0]
     return (
         not child.prefix.strip()

tests/data/cases/preview_dummy_implementations.py

@@ -48,6 +48,9 @@ def b(arg: Union[int, str, object]) -> Union[int, str]:
         raise TypeError
     return arg
 
+def has_comment():
+    ...  # not a dummy
+
 # output
 
 from typing import NoReturn, Protocol, Union, overload
@@ -98,3 +101,7 @@ def b(arg: Union[int, str, object]) -> Union[int, str]:
     if not isinstance(arg, (int, str)):
         raise TypeError
     return arg
+
+
+def has_comment():
+    ...  # not a dummy