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/resources/test/fixtures/ruff/docstring.py

@@ -93,6 +93,10 @@ 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