[ty] Detect illegal non-enum attribute accesses in Literal annotation

[sharkdp]

Summary

Detect illegal attribute accesses in Literal[X.Y] annotations if X is not an enum class.

Test Plan

New Markdown test

[github-actions]

mypy_primer results

No ecosystem changes detected ✅
No memory usage changes detected ✅

[AlexWaygood]

+ error[invalid-type-form] bidict/_typing.py:22:28: Type arguments for `Literal` must be `None`, a literal value (int, bool, str, or bytes), or an enum value

I see the error has gone away from the primer diff on the latest version of this PR, but FWIW I think "enum member" would be clearer than "enum value" in this error message. (I know that this issue predates this PR!)

[AlexWaygood]

If I apply this diff to your branch then I get some pull-types panics on the literal.md mdtest:

diff --git a/crates/ty_python_semantic/resources/mdtest/annotations/literal.md b/crates/ty_python_semantic/resources/mdtest/annotations/literal.md
index 0c266868e1..6e480724cb 100644
--- a/crates/ty_python_semantic/resources/mdtest/annotations/literal.md
+++ b/crates/ty_python_semantic/resources/mdtest/annotations/literal.md
@@ -5,7 +5,7 @@
 ## Parameterization
 
 ```py
-from typing import Literal
+from typing import Literal, Any
 from enum import Enum
 
 mode: Literal["w", "r"]
@@ -57,9 +57,12 @@ invalid4: Literal[
 
 class NotAnEnum:
     x: int = 1
+    y: list[Any] = []
 
 # error: [invalid-type-form]
 invalid5: Literal[NotAnEnum.x]
+invalid6: Literal[NotAnEnum.y[0]]
+invalid7: Literal[NotAnEnum["x"].value]
 ```

[sharkdp]

I think "enum member" would be clearer than "enum value" in this error message

👍 #19482

[sharkdp]

If I apply this diff to your branch then I get some pull-types panics on the literal.md mdtest:

I can't see how this would be related to this PR, and in fact, I can also reproduce this on main. Will look into it or open a ticket.