Byte strings aren't docstrings (#8350)

We previously incorrectly treated byte strings in docstring position as
docstrings because black does so
(https://github.com/astral-sh/ruff/pull/8283#discussion_r1375682931,
https://github.com/psf/black/issues/4002), even CPython doesn't
recognize them:

```console
$ python3.12
Python 3.12.0 (main, Oct  6 2023, 17:57:44) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def f():
...     b""" a"""
...
>>> print(str(f.__doc__))
None
```

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crates/ruff_python_formatter/tests/snapshots/format@docstring.py.snap

@@ -99,6 +99,10 @@ def docstring_that_ends_with_quote_and_a_line_break3():
     """
 
 
+class ByteDocstring:
+    b"""   has leading whitespace"""
+    first_statement = 1
+
 class TabbedIndent:
 	def tabbed_indent(self):
 		"""check for correct tabbed formatting
@@ -213,6 +217,11 @@ def docstring_that_ends_with_quote_and_a_line_break3():
     """he said "the news of my death have been greatly exaggerated" """
 
 
+class ByteDocstring:
+    b"""   has leading whitespace"""
+    first_statement = 1
+
+
 class TabbedIndent:
     def tabbed_indent(self):
         """check for correct tabbed formatting
@@ -327,6 +336,11 @@ def docstring_that_ends_with_quote_and_a_line_break3():
   """he said "the news of my death have been greatly exaggerated" """
 
 
+class ByteDocstring:
+  b"""   has leading whitespace"""
+  first_statement = 1
+
+
 class TabbedIndent:
   def tabbed_indent(self):
     """check for correct tabbed formatting
@@ -441,6 +455,11 @@ def docstring_that_ends_with_quote_and_a_line_break3():
 	"""he said "the news of my death have been greatly exaggerated" """
 
 
+class ByteDocstring:
+	b"""   has leading whitespace"""
+	first_statement = 1
+
+
 class TabbedIndent:
 	def tabbed_indent(self):
 		"""check for correct tabbed formatting
@@ -555,6 +574,11 @@ def docstring_that_ends_with_quote_and_a_line_break3():
 	"""he said "the news of my death have been greatly exaggerated" """
 
 
+class ByteDocstring:
+	b"""   has leading whitespace"""
+	first_statement = 1
+
+
 class TabbedIndent:
 	def tabbed_indent(self):
 		"""check for correct tabbed formatting