Produce equivalent code for docstrings containing backslash followed by whitespace(s) before newline (#4008)

Fixes #3727

CHANGES.md

@@ -8,7 +8,8 @@
 
 ### Stable style
 
-<!-- Changes that affect Black's stable style -->
+- Fix a crash when whitespace(s) followed a backslash before newline in a docstring
+  (#4008)
 
 ### Preview style
 

src/black/linegen.py

@@ -2,6 +2,7 @@
 Generating lines of code.
 """
 
+import re
 import sys
 from dataclasses import replace
 from enum import Enum, auto
@@ -420,7 +421,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
         if Preview.hex_codes_in_unicode_sequences in self.mode:
             normalize_unicode_escape_sequences(leaf)
 
-        if is_docstring(leaf) and "\\\n" not in leaf.value:
+        if is_docstring(leaf) and not re.search(r"\\\s*\n", leaf.value):
             # We're ignoring docstrings with backslash newline escapes because changing
             # indentation of those changes the AST representation of the code.
             if self.mode.string_normalization:

tests/data/cases/docstring.py

@@ -221,6 +221,12 @@ def stable_quote_normalization_with_immediate_inner_single_quote(self):
     '''
 
 
+def foo():
+    """
+    Docstring with a backslash followed by a space\ 
+    and then another line
+    """
+
 # output
 
 class MyClass:
@@ -442,3 +448,10 @@ def stable_quote_normalization_with_immediate_inner_single_quote(self):
 
     <text here, since without another non-empty line black is stable>
     """
+
+
+def foo():
+    """
+    Docstring with a backslash followed by a space\ 
+    and then another line
+    """