[red-knot] Preliminary tests for typing.Final

Author: sharkdp

crates/red_knot_python_semantic/resources/mdtest/type_qualifiers/final.md

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+# `typing.Final`
+
+[`typing.Final`] is a type qualifier that is used to indicate that a symbol may not be reassigned in
+any scope. Final names declared in class scopes cannot be overridden in subclasses.
+
+## Basic
+
+```py path=mod.py
+from typing import Final, Annotated
+
+FINAL_A: int = 1
+FINAL_B: Annotated[Final[int], "the annotation for FINAL_B"] = 1
+FINAL_C: Final[Annotated[int, "the annotation for FINAL_C"]] = 1
+FINAL_D: Final = 1
+FINAL_E: "Final[int]" = 1
+
+reveal_type(FINAL_A)  # revealed: Literal[1]
+reveal_type(FINAL_B)  # revealed: Literal[1]
+reveal_type(FINAL_C)  # revealed: Literal[1]
+reveal_type(FINAL_D)  # revealed: Literal[1]
+reveal_type(FINAL_E)  # revealed: Literal[1]
+
+# TODO: All of these should be errors:
+FINAL_A = 2
+FINAL_B = 2
+FINAL_C = 2
+FINAL_D = 2
+FINAL_E = 2
+```
+
+Public types:
+
+```py
+from mod import FINAL_A, FINAL_B, FINAL_C, FINAL_D, FINAL_E
+
+# TODO: All of these should be Literal[1]
+reveal_type(FINAL_A)  # revealed: int
+reveal_type(FINAL_B)  # revealed: int
+reveal_type(FINAL_C)  # revealed: int
+reveal_type(FINAL_D)  # revealed: Unknown
+reveal_type(FINAL_E)  # revealed: int
+```
+
+## Too many arguments
+
+```py
+from typing import Final
+
+class C:
+    # error: [invalid-type-form] "Type qualifier `typing.Final` expects exactly one type parameter"
+    x: Final[int, str] = 1
+```
+
+## Illegal `Final` in type expression
+
+```py
+from typing import Final
+
+class C:
+    # error: [invalid-type-form] "Type qualifier `typing.Final` is not allowed in type expressions (only in annotation expressions)"
+    x: Final | int
+
+    # error: [invalid-type-form] "Type qualifier `typing.Final` is not allowed in type expressions (only in annotation expressions)"
+    y: int | Final[str]
+```
+
+## No assignment
+
+```py
+from typing import Final
+
+DECLARED_THEN_BOUND: Final[int]
+DECLARED_THEN_BOUND = 1
+```
+
+## No assignment for bare `Final`
+
+```py
+from typing import Final
+
+# TODO: This should be an error
+NO_RHS: Final
+```
+
+[`typing.final`]: https://docs.python.org/3/library/typing.html#typing.Final