Fix bytes strings being treated as docstrings

Fixes #4002

I think this only fixes crashes so it doesn't need to go through the preview style,
but not 100% sure.

CHANGES.md

@@ -8,7 +8,7 @@
 
 ### Stable style
 
-<!-- Changes that affect Black's stable style -->
+- Fix crash on formatting bytes strings that look like docstrings (#4003)
 
 ### Preview style
 

src/black/nodes.py

@@ -14,7 +14,7 @@
 
 from black.cache import CACHE_DIR
 from black.mode import Mode, Preview
-from black.strings import has_triple_quotes
+from black.strings import get_string_prefix, has_triple_quotes
 from blib2to3 import pygram
 from blib2to3.pgen2 import token
 from blib2to3.pytree import NL, Leaf, Node, type_repr
@@ -525,6 +525,13 @@ def is_arith_like(node: LN) -> bool:
 
 
 def is_docstring(leaf: Leaf) -> bool:
+    if leaf.type != token.STRING:
+        return False
+
+    prefix = get_string_prefix(leaf.value)
+    if "b" in prefix or "B" in prefix:
+        return False
+
     if prev_siblings_are(
         leaf.parent, [None, token.NEWLINE, token.INDENT, syms.simple_stmt]
     ):

tests/data/cases/bytes_docstring.py

@@ -0,0 +1,34 @@
+def bitey():
+    b" not a docstring"
+
+def bitey2():
+    b' also not a docstring'
+
+def triple_quoted_bytes():
+    b""" not a docstring"""
+
+def triple_quoted_bytes2():
+    b''' also not a docstring'''
+
+def capitalized_bytes():
+    B" NOT A DOCSTRING"
+
+# output
+def bitey():
+    b" not a docstring"
+
+
+def bitey2():
+    b" also not a docstring"
+
+
+def triple_quoted_bytes():
+    b""" not a docstring"""
+
+
+def triple_quoted_bytes2():
+    b""" also not a docstring"""
+
+
+def capitalized_bytes():
+    b" NOT A DOCSTRING"