python: Fix highlighting for forward references (#20766)

Jaagup Averin created

[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.

Change summary

crates/languages/src/python/highlights.scm | 6 ++++++
1 file changed, 6 insertions(+)

Detailed changes