enable long_docstring_quotes_on_newline

src/black/linegen.py

@@ -396,7 +396,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
             quote = quote_char * quote_len
 
             # It's invalid to put closing single-character quotes on a new line.
-            if Preview.long_docstring_quotes_on_newline in self.mode and quote_len == 3:
+            if quote_len == 3:
                 # We need to find the length of the last line of the docstring
                 # to find if we can add the closing quotes to the line without
                 # exceeding the maximum line length.

src/black/mode.py

@@ -150,7 +150,6 @@ class Preview(Enum):
     """Individual preview style features."""
 
     annotation_parens = auto()
-    long_docstring_quotes_on_newline = auto()
     # NOTE: string_processing requires wrap_long_dict_values_in_parens
     # for https://github.com/psf/black/issues/3117 to be fixed.
     string_processing = auto()