python: Highlight attribute docstrings (#20763)

Adds more docstring highlights missing from #20486.
[PEP257](https://peps.python.org/pep-0257/) defines attribute docstrings
as
> String literals occurring immediately after a simple assignment at the
top level of a module, class, or __init__ method are called “attribute
docstrings”.

This PR adds `@string.doc` for such cases.
Before:

![Screenshot_20241116_162257](https://github.com/user-attachments/assets/6b471cff-717e-4755-9291-d596da927dc6)
After:

![Screenshot_20241116_162457](https://github.com/user-attachments/assets/96674157-9c86-45b6-8ce9-e433ca0ae8ea)

Release Notes:

- Added Python syntax highlighting for attribute docstrings.

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

crates/languages/src/python/highlights.scm

@@ -98,6 +98,25 @@
   (parameters)?
   body: (block (expression_statement (string) @string.doc)))
 
+(module
+  (expression_statement (assignment))
+  . (expression_statement (string) @string.doc))
+
+(class_definition
+  body: (block
+    (expression_statement (assignment))
+    . (expression_statement (string) @string.doc)))
+
+(class_definition
+  body: (block
+    (function_definition
+      name: (identifier) @function.method.constructor
+      (#eq? @function.method.constructor "__init__")
+      body: (block
+        (expression_statement (assignment))
+        . (expression_statement (string) @string.doc)))))
+
+
 [
   "-"
   "-="