python: Fix highlighting for forward references (#20766)

[PEP484](https://peps.python.org/pep-0484/) defines "Forward references"
for undefined types. This PR treats such annotations as types rather
than strings.
Release Notes:

- Added Python syntax highlighting for forward references.

crates/languages/src/python/highlights.scm

@@ -18,6 +18,12 @@
   (tuple (identifier) @type)
 )
 
+; Forward references
+(type
+  (string) @type
+)
+
+
 ; Function calls
 
 (decorator