fix crashes on docstring whitespace changes (#1417)

Fixes #1415

src/black/__init__.py

@@ -6006,13 +6006,14 @@ def _stringify_ast(
 
         else:
             # Constant strings may be indented across newlines, if they are
-            # docstrings; fold spaces after newlines when comparing
+            # docstrings; fold spaces after newlines when comparing. Similarly,
+            # trailing and leading space may be removed.
             if (
                 isinstance(node, ast.Constant)
                 and field == "value"
                 and isinstance(value, str)
             ):
-                normalized = re.sub(r"\n[ \t]+", "\n ", value)
+                normalized = re.sub(r" *\n[ \t]+", "\n ", value).strip()
             else:
                 normalized = value
             yield f"{'  ' * (depth+2)}{normalized!r},  # {value.__class__.__name__}"

tests/data/docstring.py

@@ -1,5 +1,5 @@
 class MyClass:
-  """Multiline
+  """ Multiline
   class docstring
   """
 
@@ -11,7 +11,7 @@ def method(self):
 
 
 def foo():
-  """This is a docstring with
+  """This is a docstring with             
   some lines of text here
   """
   return
@@ -66,6 +66,13 @@ def over_indent():
     """
   pass
 
+
+def single_line():
+    """But with a newline after it!
+
+    """
+    pass
+
 # output
 
 class MyClass:
@@ -136,3 +143,8 @@ def over_indent():
       - And the closing quote is too deep
     """
     pass
+
+
+def single_line():
+    """But with a newline after it!"""
+    pass